Method and system for programming, calibrating and/or compensating, and driving an LED display

ABSTRACT

A method and system for programming, calibrating and driving a light emitting device display, and for operating a display at a constant luminance even as some of the pixels in the display are degraded over time. The system may include extracting a time dependent parameter of a pixel for calibration. Each pixel in the display is configured to emit light when a voltage is supplied to the pixel&#39;s driving circuit, which causes a current to flow through a light emitting element. Degraded pixels are compensated by supplying their respective driving circuits with greater voltages. The display data is scaled by a compression factor less than one to reserve some voltage levels for compensating degraded pixels. As pixels become more degraded, and require additional compensation, the compression factor is decreased to reserve additional voltage levels for use in compensation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is

(A) a continuation-in-part of Ser. No. 14/738,393, filed Jun. 12, 2015,which is

(A-1) a continuation-in-part of Ser. No. 14/643,584, filed Mar. 10,2015, which is a continuation of U.S. patent application Ser. No.14/157,031, filed Jan. 16, 2014, now issued as U.S. Pat. No. 8,994,625,which is a continuation of U.S. patent application Ser. No. 13/568,784,filed Aug. 7, 2012, now issued as U.S. Pat. No. 8,736,524, which is acontinuation of U.S. patent application Ser. No. 12/571,968, filed Oct.1, 2009, now issued as U.S. Pat. No. 8,259,044, which is a continuationof U.S. patent application Ser. No. 11/304,162, filed Dec. 15, 2005, nowissued as U.S. Pat. No. 7,619,597, which claims priority pursuant to 35U.S.C. § 119 to (1) Canadian Patent No. 2,490,860, filed Dec. 15, 2004,and to (2) Canadian Patent No. 2,503,237, filed Apr. 8, 2005, and to (3)Canadian Patent No. 2,509,201, filed Jun. 8, 2005, and to (4) CanadianPatent No. 2,521,986, filed Oct. 17, 2005, and(A-2) application Ser. No. 14/738,393 is also a continuation-in-part ofSer. No. 13/898,940, filed May 21, 2013, which is a continuation of U.S.application Ser. No. 12/946,601, filed Nov. 15, 2010, which is acontinuation-in-part of prior application Ser. No. 11/402,624, filedApr. 12, 2006, now U.S. Pat. No. 7,868,857, which claims priority toCanadian Patent No. 2,504,571, filed Apr. 12, 2005; and(B) this application is also a continuation-in-part of Ser. No.12/946,601, which is a continuation-in-part of Ser. No. 11/402,624, nowissued as U.S. Pat. No. 7,868,857; and(C) this application is also a continuation-in-part of Ser. No.14/135,789, which is a continuation-in-part of Ser. No. 12/946,601,which is a continuation-in-part of Ser. No. 11/402,624, now issued asU.S. Pat. No. 7,868,857, all of which are incorporated herein byreference in their respective entireties.

FIELD OF DISCLOSURE

The present disclosure relates to display technologies, morespecifically methods and systems for programming, calibrating anddriving a light emitting device display, and compensating fornon-uniformities of elements in light emitting device displays.

BACKGROUND OF THE DISCLOSURE

Recently active-matrix organic light-emitting diode (AMOLED) displayswith amorphous silicon (a-Si), poly-silicon, organic, or other drivingbackplane have become more attractive due to advantages over activematrix liquid crystal displays. For example, the advantages include:with a-Si besides its low temperature fabrication that broadens the useof different substrates and makes feasible flexible displays, its lowcost fabrication, high resolution, and a wide viewing angle.

An AMOLED display includes an array of rows and columns of pixels, eachhaving an organic light-emitting diode (OLED) and backplane electronicsarranged in the array of rows and columns. Since the OLED is a currentdriven device, the pixel circuit of the AMOLED should be capable ofproviding an accurate and constant drive current.

U.S. Pat. No. 6,594,606 discloses a method and system for calibratingpassive pixels. U.S. Pat. No. 6,594,606 measures data line voltage anduses the measurement for pre-charge. However, this technique does notprovide the accuracy needed for active matrix, since the active matrixcalibration should work for both backplane aging and OLED aging.Further, after pre-charge, current programming must be performed.Current-programming of current driven pixels is slow due to parasiticline capacitances and suffers from non-uniformity for large displays.The speed may be an issue when programming with small currents.

Other compensation techniques have been introduced. However, there isstill a need to provide a method and system which is capable ofproviding constant brightness, achieving high accuracy and reducing theeffect of the aging of the pixel circuit.

Amorphous silicon is, for example, a promising material for AMOLEDdisplays, due to its low cost and vast installed infrastructure fromthin film transistor liquid crystal display (TFTLCD) fabrication.

All AMOLED displays, regardless of backplane technology used, exhibitdifferences in luminance on a pixel to pixel basis, primarily as aresult of process or construction inequalities, or from aging caused byoperational use over time. Luminance non-uniformities in a display mayalso arise from natural differences in chemistry and performance fromthe OLED materials themselves. These non-uniformities must be managed bythe AMOLED display electronics in order for the display device to attaincommercially acceptable levels of performance for mass-market use.

FIG. 1B illustrates an operational flow of a conventional AMOLED display10. Referring to FIG. 1B, a video source 12 contains luminance data foreach pixel and sends the luminance data in the form of digital data 14to a digital data processor 16. The digital data processor 16 mayperform some data manipulation functions, such as scaling the resolutionor changing the color of the display. The digital data processor 16sends digital data 18 to a data driver integrated circuit (IC) 20. Thedata driver IC 20 converts that digital data 18 into an analog voltageor current 22, which is sent to thin film transistors (TFTs) 26 in apixel circuit 24. The TFTs 26 convert that voltage or current 22 intoanother current 28 which flows through an organic light-emitting diode(OLED) 30. The OLED 30 converts the current 28 into visible light 36.The OLED 30 has an OLED voltage 32, which is the voltage drop across theOLED. The OLED 30 also has an efficiency 34, which is a ratio of theamount of light emitted to the current through the OLED.

The digital data 14, analog voltage/current 22, current 28, and visiblelight 36 all contain the exact same information (i.e. luminance data).They are simply different formats of the initial luminance data thatcame from the video source 12. The desired operation of the system isfor a given value of luminance data from the video source 12 to alwaysresult in the same value of the visible light 36.

However, there are several degradation factors which may cause errors onthe visible light 36. With continued usage, the TFTs will output lowercurrent 28 for the same input from the data driver IC 20. With continuedusage, the OLED 30 will consume greater voltage 32 for the same inputcurrent. Because the TFT 26 is not a perfect current source, this willactually reduce the input current 28 slightly. With continued usage, theOLED 30 will lose efficiency 34, and emit less visible light for thesame current.

Due to these degradation factors, the visible light output 36 will beless over time, even with the same luminance data being sent from thevideo source 12. Depending on the usage of the display, different pixelsmay have different amounts of degradation.

Therefore, there will be an ever-increasing error between the requiredbrightness of some pixels as specified by the luminance data in thevideo source 12, and the actual brightness of the pixels. The result isthat the decreased image will not show properly on the display.

One way to compensate for these problems is to use a feedback loop. FIG.2B illustrates an operational flow of a conventional AMOLED display 40that includes the feedback loop. Referring to FIG. 2B, a light detector42 is employed to directly measure the visible light 36. The visiblelight 36 is converted into a measured signal 44 by the light detector42. A signal converter 46 converts the measured visible light signal 44into a feedback signal 48. The signal converter 46 may be ananalog-to-digital converter, a digital-to-analog converter, amicrocontroller, a transistor, or another circuit or device. Thefeedback signal 48 is used to modify the luminance data at some pointalong its path, such as an existing component (e.g. 12, 16, 20, 26, 30),a signal line between components (e.g. 14, 18, 22, 28, 36), orcombinations thereof.

Some modifications to existing components, and/or additional circuitsmay be required to allow the luminance data to be modified based on thefeedback signal 48 from the signal converter 46. If the visible light 36is lower than the desired luminance from video source 12, the luminancesignal may be increased to compensate for the degradation of the TFT 26or the OLED 30. This results in that the visible light 36 will beconstant regardless of the degradation. This compensation scheme isoften known as Optical Feedback (OFB). However, in the system of FIG.2B, the light detector 42 must be integrated onto a display, usuallywithin each pixel and coupled to the pixel circuitry. Not consideringthe inevitable issues of yield when integrating a light detector intoeach pixel, it is desirable to have a light detector which does notdegrade itself, however such light detectors are costly to implement,and not compatible with currently installed TFT-LCD fabricationinfrastructure.

Therefore, there is a need to provide a method and system which cancompensate for non-uniformities in displays without measuring a lightsignal.

AMOLED displays are conventionally operated according to digital datafrom a video source. The OLEDs within the display can be programmed toemit light with luminance according to a programming voltage or aprogramming current. The programming current or programming voltage areconventionally set by a display driver that takes digital data as inputand has an analog output for sending the programming current orprogramming voltage to pixel circuits. The pixel circuits are configuredto drive current through OLEDs based on the programming current orprogramming voltage.

SUMMARY OF ASPECTS OF THE PRESENT DISCLOSURE

It is an object of the invention to provide a method and system thatobviates or mitigates at least one of the disadvantages of existingsystems.

According to an aspect of the present disclosure, a method and systemare provided for programming, calibrating and driving a light emittingdevice display, and for operating a display at a constant luminance evenas some of the pixels in the display are degraded over time. The systemmay include extracting a time dependent parameter of a pixel forcalibration. Each pixel in the display is configured to emit light whena voltage is supplied to the pixel's driving circuit, which causes acurrent to flow through a light emitting element. Degraded pixels arecompensated by supplying their respective driving circuits with greatervoltages. The display data is scaled by a compression factor less thanone to reserve some voltage levels for compensating degraded pixels. Aspixels become more degraded, and require additional compensation, thecompression factor is decreased to reserve additional voltage levels foruse in compensation.

In accordance with an aspect of the present invention there is provideda method of real-time calibration for a display array having a pluralityof pixel circuits arranged in row and column, including the steps of:generating a priority list of pixels, which is used to prioritize pixelsfor calibration based on display and previous calibration data, thepriority list being used to select one or more (n) pixels which areprogrammed with currents higher than a threshold current forcalibration; selecting n pixels in a selected column of the displayarray from the linked list; implementing programming to the pixels inthe selected column, including: monitoring a pixel current for the npixels and obtaining calibration data; updating a compensation memorybased on the calibration data for calibration; sorting the priority listfor the next programming.

In accordance with a further aspect of the present invention there isprovided a system for real-time calibration for a display array having aplurality of pixel circuits arranged in row and column, each pixelcircuit having a light emitting device and a driving transistor, thesystem including: a calibration scheduler for controlling programmingand calibration of the display array, including: a priority list forlisting one or more pixels for calibration based on display data; modulefor enabling, during a programming cycle, calibration mode for one ormore pixels in the selected column, which are selected from the prioritylist, and during a programming cycle, enabling normal operation mode forthe rest of the pixels in the selected column; a monitor for monitoringa pixel current for the pixels in the calibration mode through theselected column; a generator for generating a calibration data based onthe monitoring result; a memory for storing calibration data; and anadjuster for adjusting a programming data applied to the display arraybased on the calibration data when the pixel on the normal operationmode is programmed.

In accordance with a further aspect of the present invention there isprovided a system for a display array having a pixel circuit, the pixelcircuit being programmed through a data line, the system including: adata source for providing a programming data into the pixel circuit; acurrent-controlled voltage source associated with the voltage source forconverting a current on the data line to a voltage associated with thecurrent to extract a time dependent parameter of the pixel circuit.

In accordance with a further aspect of the present invention there isprovided a system for a display array including a plurality of pixelcircuits, each pixel circuit including a driving transistor, at leastone switch transistor, a storage capacitor and a light emitting device,the system including: a monitor for monitoring a current or voltage onthe pixel circuit; a data process unit for controlling the operation ofthe display array, the data process unit extracting information on anaging of the pixel circuit, based on the monitored current or voltageand determining a state of the pixel circuit; a driver controlled by thedata process unit and for providing programming and calibration data tothe pixel circuit, based on the state of the pixel circuit.

In accordance with a further aspect of the present invention there isprovided a method of driving a display array, the display arrayincluding a plurality of pixel circuits, each pixel circuit including adriving transistor, at least one switch transistor, a storage capacitorand a light emitting device, the method including the steps of: applyinga current or voltage to the pixel circuit; monitoring a current orvoltage flowing through the pixel circuit; extracting information on anaging of the pixel circuit, based on the monitored current or voltageand determining the state of the pixel circuit; providing operationvoltage to the pixel circuit, including determining programming andcalibration data for the pixel circuit based on the state of the pixelcircuit.

In accordance with a further aspect of the present invention there isprovided a method of driving a display array, the display arrayincluding a plurality of pixel circuits, each pixel circuit including adriving transistor, at least one switch transistor, a storage capacitorand a light emitting device, the method including the steps of: applyinga current or voltage to the light emitting device; monitoring a currentor voltage flowing through the light emitting device; predicting a shiftin the voltage of the light emitting device, based on the monitoredcurrent or voltage and determining the state of the pixel circuit; andproviding, to the light emitting device, a bias associated with theshift in the voltage of the light emitting device.

In accordance with a further aspect of the present invention there isprovided a system for driving a display array, the display arrayincluding a plurality of pixel circuits, each pixel circuit including adriving transistor, at least one switch transistor, a storage capacitorand a light emitting device, the system including: a monitor formonitoring a current or voltage on the pixel circuit; a data processunit for predicting a shift in the voltage of the light emitting device,based on the monitored current or voltage and determining the state ofthe pixel circuit; and a circuit for providing, to the light emittingdevice, a bias associated with the shift in the voltage of the lightemitting device.

In accordance with an aspect of the present invention there is provideda system for a display array including a plurality of pixel circuits,each pixel circuit having a driving transistor, at least one switchtransistor, a storage capacitor and a light emitting device, the lightemitting device being located at a programming path for programming thepixel circuit, the system including: a controller for controlling theoperation of the display array; a driver for providing operation voltageto the pixel circuit based on the control of the controller; and thedriver providing the operation voltage to the pixel circuit during aprogramming cycle such that the light emitting device being removed fromthe programming path.

It is an object of the invention to provide a method and system thatobviates or mitigates at least one of the disadvantages of existingsystems.

In accordance with an aspect of the present invention there is provideda system for compensating non-uniformities in a light emitting devicedisplay which includes a plurality of pixels and a source for providingpixel data to each pixel circuit. The system includes: a module formodifying the pixel data applied to one or more than one pixel circuit,an estimating module for estimating a degradation of a first pixelcircuit based on measurement data read from a part of the first pixelcircuit, and a compensating module for correcting the pixel data appliedto the first or a second pixel circuit based on the estimation of thedegradation of the first pixel circuit.

According to a further aspect of the present disclosure, a system andmethod are disclosed for operating a display at a constant luminanceeven as some of the pixels in the display are degraded over time. Eachpixel in the display is configured to emit light when a voltage issupplied to the pixel's driving circuit, which causes a current to flowthrough a light emitting element. Degraded pixels are compensated bysupplying their respective driving circuits with greater voltages. Thedisplay data is scaled by a compression factor less than one to reservesome voltage levels for compensating degraded pixels. As pixels becomemore degraded, and require additional compensation, the compressionfactor is decreased to reserve additional voltage levels for use incompensation.

In accordance with another aspect of the present invention there isprovided a display degradation compensation system for adjusting theoperating conditions for pixels in an OLED display to compensate fornon-uniformity or aging of the display. The system includes a controllerprogrammed to set an initial value for at least one of peak luminanceand an operating condition, calculate compensation values for the pixelsin the display, determine the number of pixels having compensationvalues larger than a predetermined threshold compensation value, and ifthe determined number of pixels having compensation values larger thansaid predetermined threshold value is greater than a predeterminedthreshold number, adjust the set value until said determined number ofpixels is less than said predetermined threshold number.

According to a still further aspect of the present disclosure, a displaydegradation compensation system and method are provided for adjustingthe operating conditions for pixels in an OLED display to compensate fornon-uniformity or aging of the display. The system or method sets aninitial value for at least one of peak luminance and an operatingcondition, calculates compensation values for the pixels in the display,determines the number of pixels having compensation values larger than apredetermined threshold compensation value, and if the determined numberof pixels having compensation values larger than said predeterminedthreshold value is greater than a predetermined threshold number,adjusts the set value until said determined number of pixels is lessthan said predetermined threshold number.

In accordance with a further aspect of the present invention there isprovided a method of compensating non-uniformities in a light emittingdevice display having a plurality of pixels, including the steps of:estimating a degradation of the first pixel circuit based on measurementdata read from a part of the first pixel circuit, and correcting pixeldata applied to the first or a second pixel circuit based on theestimation of the degradation of the first pixel circuit.

The present disclosure provides a method of maintaining uniformluminosity of an AMOLED display. The AMOLED display includes an array ofpixels having light emitting devices. The light emitting devices areconfigured to emit light according to digital input from a video source.The video source includes digital data corresponding to a desiredluminance of each pixel in the AMOLED display. Over time, aspects withinthe light emitting devices and their associated driving circuits degradeand require compensation to continue to emit light with the sameluminance for a given digital input.

Degradation of the pixels in the light emitting display are compensatedby incrementing the digital inputs of the pixels according to a measuredor estimated degradation of the pixels. To allow for compensation tooccur, the digital input is compressed to a range of values less than anavailable range. Compressing the digital input is carried out accordingto a compression factor, which is a number less than one. In animplementation of the present disclosure, the digital inputs aremultiplied by the compression factor, which compresses the digital inputto a range less than the available range. The remaining portion of thedigital range can be used to provide compensation to degraded pixelsbased on measured or estimated degradation of the pixels. The presentdisclosure provides methods for setting and adjusting the compressionfactor to statically or dynamically adjust the compression factor andprovide compensation to the display by incrementing the digital signalsbefore the signals are sent to the driving circuits.

According to yet another aspect of the present disclosure, a method isprovided of compensating for a degradation of a pixel having a drivingcircuit for driving current through a light emitting device based on aninput. The method includes: receiving luminosity data; scaling theluminosity data by a compression factor to create compressed data;compensating for the degradation of the pixel by adjusting thecompressed data to create compensated data; and supplying the drivingcircuit based on the compensated data.

The scaling can be carried out by multiplying the luminosity data by aconstant integer to create resulting data with a greater number of bits,and multiplying the resulting data by the compression factor. Theluminosity data can be an eight-bit integer and the compressed data is aten-bit integer. The driving circuit can include at least one thin filmtransistor (TFT), which can be an n-type TFT. The at least one TFT canbe used to drive current through the light emitting device. Thedegradation can be due to a voltage threshold of the at least one TFT ordue to a shift in the voltage threshold of the at least one TFT.

The light emitting device can be an organic light emitting diode (OLED).The degradation can be due to a bias voltage of the OLED or due to ashift in the bias voltage of the OLED. The degradation can be due to avoltage required to compensate for an inefficiency of the OLED or due toa shift in the voltage required to compensate for the inefficiency ofthe OLED.

The compression factor can be determined based on a user selectedprofile and a usage time of the pixel. The compression factor can bedetermined based on an estimation of degradation of the pixel and on adisplay requirement. The estimation can be based on a design of hardwareaspects of the pixel and of the driving circuit.

According to a further aspect of the present disclosure, a method isdisclosed of compensating for a degradation of a pixel in a displayhaving a plurality of pixels said pixel having a driving circuit fordriving a current through a light emitting device based on an input, theinput being supplied to the driving circuit by a display driver. Themethod includes: receiving luminosity data; scaling the luminosity databy a compression factor to create compressed data; compensating for adegradation of a pixel in the display by adjusting the compressed databased on the degradation to create compensated data; and sending thecompensated data to the display driver.

The method can further include: ascertaining a maximum compensationapplied to the plurality of pixels; and adjusting the compression factorbased on the ascertained maximum compensation. The adjusting can becarried out by computing the ratio of the ascertained maximumcompensation to a maximum assignable value of the inputs and updatingthe compression factor to be one minus the computed ratio. Theluminosity data can include eight-bit integers. The scaling can becarried out by: multiplying the luminosity data by a constant integer tocreate resulting data with a greater number of bits, and multiplying theresulting data by the compression factor. At least one of the drivingcircuits can include at least one thin film transistor (TFT).

The method can further include compensating for degradations of theplurality of pixels in the display by adjusting the compressed databased on the degradations to create compensated data. The at least oneTFT can be used to drive current through at least one of the lightemitting devices. The degradation can be due to a voltage threshold ofthe at least one TFT or due to a shift in the voltage threshold of theat least one TFT.

At least one of the light emitting devices can be an organic lightemitting diode (OLED). The degradation can be due to a bias voltage ofthe OLED or due to a shift in the bias voltage of the OLED. Thedegradation can be due to a voltage required to compensate for aninefficiency of the OLED or due to a shift in the voltage required tocompensate for the inefficiency of the OLED.

The compression factor can be determined based on a user selectedprofile and a usage time of the display. The compression factor can bedetermined based on an estimation of the degradation of the display andbased on display requirements and the design of hardware aspects withinthe display.

According to yet another aspect of the present disclosure, a method isdisclosed of operating a display having a plurality of pixels tocompensate for degradation of the plurality of pixels. The plurality ofpixels have driving circuits for driving currents through light emittingdevices based on inputs. The method includes: operating the displayaccording to a first compression factor by: receiving a first set ofluminosity data for the plurality of pixels; scaling the first set ofluminosity data by the first compression factor to create a first set ofcompressed data; compensating for a first degradation of the pluralityof pixels by adjusting the first set of compressed data based on a firstset of offset increments to create a first set of compensated data; andsupplying the driving circuits based on the first set of compensateddata; determining a second compression factor based on a seconddegradation of the plurality of pixels; and operating the displayaccording to the second compression factor by: receiving a second set ofluminosity data for the plurality of pixels; scaling the second set ofluminosity data by the second compression factor to create a second setof compressed data; compensating for the second degradation of theplurality of pixels by adjusting the second set of compressed data basedon a second set of offset increments to create a second set ofcompensated data; and supplying the driving circuits based on the secondset of compensated data.

The method can further include, prior to operating the display accordingto the first compression factor, determining the first compressionfactor based on the first degradation of the plurality of pixels. Theadjusting the first set of compressed data can be carried out by addingthe first set of offset increments to the first set of compressed datato create the first set of compensated data. The adjusting the secondset of compressed data can be carried out by adding the second set ofoffset increments to the second set of compressed data to create thesecond set of compensated data. The adjusting the first set ofcompressed data can be carried out by subtracting the first set ofoffset increments from the first set of compressed data to create thefirst set of compensated data. The adjusting the second set ofcompressed data can be carried out by subtracting the second set ofoffset increments from the second set of compressed data to create thesecond set of compensated data.

The determining the first compression factor can be carried out byascertaining the maximum value in the first set of offset increments andcomputing the ratio of the ascertained maximum to a maximum assignableinput value. The first set of offset increments can be determined basedon estimates of degradation of the plurality of pixels. The determiningthe first compression factor can be carried out by ascertaining themaximum value in the first set of offset increments and computing theratio of the ascertained maximum to a maximum assignable input value.The first set of offset increments can be determined based onmeasurements of degradation of the plurality of pixels. The determiningthe second compression factor can be carried out by ascertaining themaximum value in the second set of offset increments and computing theratio of the ascertained maximum to a maximum assignable input value.The second set of offset increments can be determined based on estimatesof degradation of the plurality of pixels. The determining the secondcompression factor can be carried out by ascertaining the maximum valuein the second set of offset increments and computing the ratio of theascertained maximum to a maximum assignable input value. The second setof offset increments can be determined based on measurements ofdegradation of the plurality of pixels.

The first set of luminosity data and second set of luminosity data caninclude eight-bit integers. The scaling the first set of luminosity datacan be carried out by: multiplying the first set of luminosity data by aconstant integer to create a first set of resulting data that includesintegers having a number of bits greater than eight; and multiplying thefirst set of resulting data by the first compression factor, and whereinthe scaling the second set of luminosity data is carried out by:multiplying the second set of luminosity data by the constant integer tocreate a second set of resulting data that includes integers having anumber of bits greater than eight; and multiplying the second set ofresulting data by the second compression factor.

According to a still further aspect of the present disclosure, a displaydegradation compensation system is disclosed for compensating for adegradation of a plurality of pixels in a display. The plurality ofpixels have driving circuits for driving currents through light emittingdevices. The display degradation compensation system includes: a digitaldata processor module for receiving a luminosity data, compressing theluminosity data according to a compression factor, and compensating forthe degradation of the plurality of pixels by adjusting the compresseddata to create compensated data; and a display driver for receiving thecompensated data and supplying the inputs to the driving circuits, thedriving circuits being configured to deliver the driving currents to thelight emitting devices based on the received compensated data. Theadjusting the compressed data can be carried out according to ameasurement of the degradation of the plurality of pixels. The digitaldata processor module can include a digital adder for adjusting thecompressed data to create compensated data.

The display degradation compensation system can further include acompensation module for determining the compression factor. Thecompensation module can be configured to determine the compressionfactor according to a function including a measurement of thedegradation of the plurality of pixels. The compensation module can beconfigured to dynamically adjust the compression factor according to aninput specified by a user and according to a usage time of the display.The compensation module can be configured to dynamically adjust thecompression factor according to a function including a measurement ofthe degradation of the plurality of pixels. The digital data processormodule can be configured to receive eight-bit luminance data and outputten-bit compensated data. At least one of the light emitting devices canbe an organic light emitting diode. At least one of the driving circuitscan include at least one thin film transistor.

According to another aspect of the present disclosure, a system isdisclosed for compensating for non-uniformities in a display having aplurality of pixels. At least one of the plurality of pixels includes apixel circuit having a light emitting device. The pixel circuit can beconfigured to drive the pixel based on luminance data. The systemincludes: a module for modifying the pixel data applied to one or morethan one pixel, the module including: an estimating module forestimating a degradation of a first pixel circuit based on measurementdata read from the first pixel circuit; a grayscale compression modulefor compressing the luminance data according to a grayscale compressionalgorithm to reserve grayscale values; and a compensating module forcorrecting the compressed luminance data applied to the first or asecond pixel circuit based on the estimation of the degradation of thefirst pixel circuit; and a display driver for receiving the correctedluminance data and supplying the pixel circuit with an analog voltage orcurrent based on the corrected luminance data.

The grayscale compression module can transform the luminance data so asto use luminance values less than those of the original luminance data.The luminance data can be eight-bit data. The compressing can be carriedout in the grayscale compression module to transform the luminance datato a range of 200 values.

The reserved grayscale values can be reserved at a high end of anavailable range to allow for providing corrections to the compressedluminance data that increase the luminosity of corrected pixels. Thereserved grayscale values can be reserved at a low end of an availablerange to allow for providing corrections to the compressed luminancedata that decrease the luminosity of corrected pixels.

The compensating module can correct the luminance data according to adecreasing brightness algorithm. The compensating module can correct theluminance data according to a constant brightness algorithm.

This summary of the invention does not necessarily describe all featuresof the invention.

Other aspects and features of the present invention will be readilyapparent to those skilled in the art from a review of the followingdetailed description of preferred embodiments in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings wherein:

FIG. 1A is a flow chart showing a process for calibration-scheduling inaccordance with an embodiment of the present invention;

FIG. 2A is a diagram showing an example of a system structure forimplementing the calibration-scheduling of FIG. 1A;

FIG. 3A is a diagram showing a system architecture for avoltage-extracting, programming and driving in accordance with anembodiment of the present invention;

FIG. 4A is a diagram showing an example of the extracting, programmingand driving system of FIG. 3A and a pixel circuit;

FIG. 5A is a diagram showing a further example of the extracting,programming and driving system of FIG. 3A and a pixel circuit;

FIG. 6A is a diagram showing a further example of the extracting,programming and driving system of FIG. 3A and a pixel circuit;

FIG. 7A is a diagram showing a further example of the extracting,programming and driving system of FIG. 3A and a pixel circuit;

FIG. 8A is a diagram showing a pixel circuit to which a step-calibrationdriving in accordance with an embodiment of the present invention isapplied;

FIG. 9A is a diagram showing an example of a driver and extraction blockand the driving transistor of FIG. 8A;

FIG. 10A is a diagram showing an example of an extraction algorithmimplemented by a DPU block of FIG. 9A;

FIG. 11A is a diagram showing a further example of the extractionalgorithm implemented by the DPU block of FIG. 9A;

FIG. 12A is a timing diagram showing an example of waveforms for thestep-calibration driving;

FIG. 13A is a timing diagram showing a further example of waveforms forthe step-calibration driving;

FIG. 14A is a diagram showing a pixel circuit to which thestep-calibration driving is applicable;

FIG. 15A is a graph showing the results of simulation for thestep-calibration driving;

FIG. 16A is a diagram showing an example of a system architecture forthe step-calibration driving with a display array;

FIG. 17A is a timing diagram showing an example of waveforms applied tothe system architecture of FIG. 16A;

FIG. 18A is a timing diagram showing an example of waveforms for avoltage/current extraction;

FIG. 19A is a timing diagram showing a further example of waveforms forthe voltage/current extraction;

FIG. 20A is a diagram showing a pixel circuit to which thevoltage/current extraction of FIG. 19A is applicable;

FIG. 21A is a timing diagram showing a further example of waveforms forthe voltage/current extraction;

FIG. 22A is a diagram showing a pixel circuit to which thevoltage/current extraction of FIG. 21A is applicable;

FIG. 23A is a diagram showing a mirror based pixel circuit to which OLEDremoving in accordance with an embodiment of the present invention isapplied;

FIG. 24A is a diagram showing a programming path of FIG. 23A whenapplying the OLED removing;

FIG. 25A is a diagram showing an example of a system architecture forthe OLED removing; and

FIG. 26A is a graph showing the simulation result for the voltage onIDATA line for different threshold voltage.

FIG. 1B illustrates a conventional AMOLED system.

FIG. 2B illustrates a conventional AMOLED system that includes a lightdetector and a feedback scheme that uses the signal from the lightdetector.

FIG. 3B illustrates a light emitting display system to which acompensation scheme in accordance with an embodiment of the presentinvention is applied.

FIG. 4B illustrates an example of the light emitting display system ofFIG. 3B.

FIG. 5B illustrates an example of a pixel circuit of FIG. 5B.

FIG. 6B illustrates a further example of the light emitting displaysystem of FIG. 3B.

FIG. 7B illustrates an example of a pixel circuit of FIG. 6B.

FIG. 8B illustrates an example of modules for the compensation schemeapplied to the system of FIG. 4B.

FIG. 9B illustrates an example of a lookup table and a compensationalgorithm module of FIG. 7B.

FIG. 10B illustrates an example of inputs to a TFT-to-pixel circuitconversion algorithm module.

FIG. 11A-1 illustrates an experimental result of a video sourceoutputting equal luminance data for each pixel for a usage time of zerohours.

FIG. 11B-1 illustrates an experimental result of a video sourceoutputting maximum luminance data to some pixels and zero luminance datato other pixels for a usage of time of 1000 hours.

FIG. 11C-1 illustrates an experimental result of a video sourceoutputting equal luminance data for each pixel after some pixelsreceived maximum luminance data and others pixels received zeroluminance data for a usage time of 1000 hours when no compensationalgorithm is applied.

FIG. 11D-1 illustrates an experimental result of a video sourceoutputting equal luminance data for each pixel after some pixelsreceived maximum luminance data and others pixels received zeroluminance data for a usage time of 1000 hours when a constant brightnesscompensation algorithm is applied.

FIG. 11E-1 illustrates an experimental result of a video sourceoutputting equal luminance data for each pixel after some pixelsreceived maximum luminance data and others pixels received zeroluminance data for a usage time of 1000 hours when a decreasingbrightness compensation algorithm is applied.

FIG. 12B illustrates an example of a grayscale compression algorithm.

FIG. 13B is a data flow chart showing the compression and compensationof luminosity input data used to drive an AMOLED display.

FIG. 14B is a flowchart illustrating a method for selecting thecompression factor according to display requirements and the design ofthe pixel circuit.

FIG. 15B is a flowchart illustrating a method for selecting thecompression factor according to a pre-determined headroom adjustmentprofile.

FIG. 16B is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a threshold over a previous compensation.

FIG. 17B is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a previously measured maximum.

FIG. 18B is a flowchart illustrating a method for periodically adjustingthe peak luminance for compensation.

FIG. 19B is a flowchart illustrating a method for periodically adjustingoperating conditions for compensation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE DISCLOSURE

Embodiments of the present invention are described using a pixelincluding a light emitting device and a plurality of transistors. Thelight emitting device may be an organic light emitting diode (OLED). Itis noted that “pixel” and “pixel circuit” may be used interchangeably.

Real-time calibration-scheduling for a display array having a pluralityof pixels is described in detail. FIG. 1A illustrates a process for acalibration-scheduling in accordance with an embodiment of the presentinvention. According to this technique, the pixels are calibrated basedon their aging and/or usage during the normal operation of the displayarray.

A linked list of pixels is generated in step S2. The linked listcontains an identification of a pixel with high brightness forcalibration. The linked list is used to schedule the priority incalibration.

In step S4, “n” is chosen based on the display size and expectedinstability with time (e.g. shift in characteristics of transistors andlight emitting device). “n” represents the number of pixels that arecalibrated in each programming cycle. “n” may be one or more than one.

Then programming cycle starts at step S6. The step S6 includes stepsS8-S16. The steps S8-S16 are implemented on a selected column of thedisplay array.

In step S8, “n” pixels in the selected column are selected from thebeginning of the linked list, hereinafter referred to as “SelectedPixels”.

In step S10, “Calibration Mode” is enabled for the Selected Pixels, and“Normal Operation Mode” is enabled for the rest of the pixels in theselected column of the display array.

In step S12, all pixels in the selected column are programmed by avoltage source driver (e.g. 28 of FIG. 2A) which is connected to a dataline of the pixel.

For the Selected Pixels, current flowing through the data line ismonitored during the programming cycle. For the pixels other than theSelected Pixels in the selected column, the corresponding programmingvoltage is boosted using data stored in a memory (e.g. 34 of FIG. 2A),hereinafter referred to as “ΔV compensation memory”.

In step S14, the monitored current is compared with the expected currentthat must flow through the data line. Then, a calibration data curve forthe Selected Pixels is generated. The AV compensation memory is updatedbased on the calibration data curve.

The calibration data curve stored in the ΔV compensation memory for apixel will be used to boost programming voltage for that pixel in thenext programming cycles when that pixel is in the Normal Operation Mode.

In step S16, the identifications of the Selected Pixels are sent to theend of the linked list. The Selected Pixels have the lowest priority inthe linked list for calibration.

During display operation (S6-S16), the linked list will provide a sortedpriority list of pixels that must be calibrated. It is noted that in thedescription, the term “linked list” and the term “priority list” may beused interchangeably.

The operation goes back (S18) to the step S8. The next programming cyclestarts. A new column in the display array is activated (selected), and,new “n” pixels in the new activated column are selected from the top ofthe linked list. The ΔV compensation memory is updated using thecalibration data obtained for the new Selected Pixels.

The number of the Selected Pixels, “n”, is now described in detail. Asdescribed above, the number “n” is determined based on the display sizeand expected instability in device characteristics with time. It isassumed that the total number of pixels N is N=3×m₁×m₂, where m1 and m2are the number of rows and columns in the display, respectively.

The highest rate in characteristics shift is K (=ΔI·Δt·I). Eachprogramming cycle takes t=1/f·m₂. The maximum expected shift incharacteristics after the entire display is calibrated isΔI/I=K·t·N/n<e, where e is the allowed error. After this the calibrationcan be redone from the beginning, and the error is eliminated. Thisshows that n>K·t·N/e or n>3·K·m₁/f·e. For instance, if K=1%/hr, m₁=1024,f=60 Hz, and e=0.1%, then n>0.14, which implies that it is needed tocalibrate once in 5 programming cycles. This is achievable with onecalibration unit, which operates only one time in 5 programming cycles.Each calibration unit enables calibration of one pixel at a programmingcycle. If e=0.01%, n>1.4. This means that two calibration unitscalibrating two pixels in each programming cycle are required. Thisshows that it is feasible to implement this calibration system with verylow cost.

The frequency of calibration can be reduced automatically as the displayages, since shifts in characteristics will become slower as the timeprogresses. In addition, the pixels that are selected for calibrationcan be programmed with different currents depending on display data. Theonly condition is that their programming current is larger than areference current. Therefore, the calibration can be performed atmultiple brightness levels for one pixel to achieve higher accuracy.

The linked list is described in detail. In the linked list, the pixelswith high brightness for calibration are listed. The display data isused to determine the pixels with high brightness for calibration.Calibration at low currents is slow and often not accurate. In addition,maximum shift in characteristics occurs for pixels with high current.Thus, in order to improve the accuracy and speed of calibration, thepixels, which must be programmed with currents higher than a thresholdcurrent I_(TH), are selected and stored in the linked list.

I_(TH) is a variable and may be “0”. For I_(TH)=0, all pixels are listedin the linked list, and the calibration is performed for all pixelsirrespective of their programming current.

The calibration-scheduling technique described above is applicable toany current programmed pixels, for example, but not limited to, acurrent mirror based pixel.

FIG. 2A illustrates an example of a system structure for implementingthe calibration-scheduling of FIG. 1A. A system 30 of FIG. 2A forimplementing calibration-scheduling algorithm is provided to a displayarray 10 having a plurality of pixel circuits 12. The pixel circuit 12is a current programmed pixel circuit, such as, but not limited to acurrent mirror based pixel. The pixel circuits 12 are arranged in rowand column.

The pixel circuit 12 may include an OLED and a plurality of transistors(e.g. TFTs). The transistor may be fabricated using amorphous silicon,nano/micro crystalline silicon, poly silicon, organic semiconductorstechnologies (e.g. organic TFT), NMOS/PMOS technology or CMOS technology(e.g. MOSFET). The display array 10 may be an AMOLED display array.

The pixel circuit 12 is operated by a gate line 14 connected to a gatedriver 20, a data line 16 connected to a voltage data driver 28, and apower line connected to a power supply 24. In FIG. 2A, two data lines,two gate lines and two power lines are shown as an example. It isapparent that more than two data lines, two gate lines and two powerlines may be provided to the display array 10.

The system 30 includes a calibration scheduler and memory block 32 forcontrolling programming and calibration of the display array 10, and aΔV compensation memory 34 for storing ΔV compensation voltage (value).In each programming cycle, a column of the display array 10 is selected.The calibration scheduler and memory block 32 enables Normal OperationMode or Calibration Mode for the selected column (i.e., data line)during that programming cycle.

The system 30 further includes a monitoring system for monitoring andmeasuring a pixel current. The monitoring system includes switches 36and 38 and a voltage sensor 40 with an accurate resistor 42. In FIG. 2A,the switches 36 and 38 are provided for each data line as an example.

The system 30 further includes a generator for generating ΔVcompensation voltage based on the monitoring result. The generatorincludes an analog/digital converter (A/D) 44, a comparator 46, and atranslator 48. The A/D 44 converts the analog output of the voltagesensor 40 into a digital output. The comparator 46 compares the digitaloutput to an output from the translator 48. The translator 48 implementsfunction f(V) on a digital data input 52. The translator 48 converts thecurrent data input 52 to the voltage data input through f(v). The resultof the comparison by the comparator 46 is stored in the ΔV compensationmemory 34.

The system 30 further includes an adder 50 for adding the digital datainput 52 and the ΔV compensation voltage stored in the ΔV compensationmemory 34. The voltage data driver 28 drives a data line based on theoutput of the adder 50. The programming data for the data line isadjusted by adding the ΔV compensation voltage.

When the calibration scheduler and memory block 32 enables the NormalOperation Mode for a selected data line, the switch 36 is activated. Thevoltage output from the voltage data driver 28 is directly applied tothe pixel on that data line.

When the calibration scheduler and memory block 32 enables theCalibration Mode for that data line, the switch 38 is activated. Thevoltage is applied to the pixel on that data line through the accurateresistor 42. The voltage drop across the resistor 42 at the final stagesof the programming time (i.e. when initial transients are finished) ismeasured by the voltage sensor 40. The voltage drop monitored by thevoltage sensor 40 is converted to digital data by the A/D 44. Theresulting value of the voltage drop is proportional to the currentflowing through the pixel if the pixel is a current programmed pixelcircuit. This value is compared by the comparator 46 to the expectedvalue obtained by the translator 48.

The difference between the expected value and the measured value isstored in the AV compensation memory 34, and will be used for asubsequent programming cycle. The difference will be used to adjust thedata voltage for programming of that pixel in future.

The calibration scheduler and memory block 32 may include the linkedlist described above. In the beginning, the linked list is generatedautomatically. It may be just a list of pixels. However, during theoperation it is modified.

The calibration of the pixel circuits with high brightness guaranteesthe high speed and accurate calibration that is needed in large or smallarea displays.

Since the display array 10 is driven using a voltage programmingtechnique, it is fast and can be used for high-resolution and large areadisplays.

Due to speed, accuracy, and ease of implementation, the applications ofthe calibration-scheduling technique ranges from electroluminescentdevices used for cellphones, personal organizers, monitors, TVs, tolarge area display boards.

The system 30 monitors and measures voltage drop which depends on timedependent parameters of the pixel, and generates a desirable programmingdata. However, the time dependent parameters of the pixel may beextracted by any mechanisms other than that of FIG. 2A.

A further technique for programming, extracting time dependentparameters of a pixel and driving the pixel is described in detail withreference to FIGS. 3A-7A. This technique includes voltage-extracting forcalibration. Programming data is calibrated with the extractedinformation, resulting in a stable pixel current over time. Using thistechnique, the aging of the pixel is extracted.

FIG. 3A illustrates a system architecture for implementing avoltage-extracting, programming and driving in accordance with anembodiment of the present invention. The system of FIG. 3A implementsthe voltage-extracting and programming to a current mode pixel circuit60. The pixel circuit 60 includes a light emitting device and aplurality of transistors having a driving transistor (not shown). Thetransistors may be TFTs.

The pixel circuit 60 is selected by a select line SEL and is driven byDATA on a data line 61. A voltage source 62 is provided to write aprogramming voltage V_(P) into the pixel circuit 60. Acurrent-controlled voltage source (CCVS) 63 having a positive node and anegative node is provided to convert the current on the data line 61 toa voltage Vext. A display controller and scheduler 64 operates the pixelcircuit 60. The display controller and scheduler 64 monitors anextracted voltage Vext output from the CCVS 63 and then controls thevoltage source 62.

The resistance of CCVS 63 is negligible. Thus the current on the dataline 61 is written as:I _(Line) =I _(piexl)β(V _(P) −V _(T))²  (1)where I_(Line) represents the current on the data line 61, I_(piexl)represents a pixel current, V_(T) represents the threshold voltage ofthe driving transistor included in the pixel circuit 60, and representsthe gain parameter in the TFT characteristics.

As the threshold voltage of the driving TFT increases during the time,the current on the data line 61 decreases. By monitoring the extractedvoltage Vext, the display controller and scheduler 64 determines theamount of shift in the threshold voltage.

The threshold voltage VT of the driving transistor can be calculate as:V _(T) =V _(P)=(I _(Line)/β)^(0.5)  (2)

The programming voltage V_(P) is modified with the extractedinformation. The extraction procedure can be implemented for one orseveral pixels during each frame time.

FIG. 4A illustrates an example of a system for the voltage-extracting,programming and driving of FIG. 3A, which is employed with atop-emission current-cell pixel circuit 70. The pixel circuit 70includes an OLED 71, a storage capacitor 72, a driving transistor 73 andswitch transistors 74 and 75.

The transistors 73, 74 and 75 may be n-type TFTs. However, thesetransistors 73, 74 and 75 may be p-type transistors. Thevoltage-extracting and programming technique applied to the pixelcircuit 70 is also applicable to a pixel circuit having p-typetransistors.

The driving transistor 73 is connected to a data line 76 through theswitch transistor 75, and is connected to the OLED 71, and also isconnected to the storage capacitor 72 through the switch transistor 74.The gate terminal of the driving transistor 73 is connected to thestorage capacitor 72. The gate terminals of the switch transistors 74and 75 are connected to a select line SEL. The OLED 71 is connected to avoltage supply electrode or line VDD. The pixel circuit 70 is selectedby the select line SEL and is driven by DATA on the data line 76.

A current conveyor (CC) 77 has X, Y and Z terminals, and is used toextract a current on the data line 76 without loading it. A voltagesource 78 applies programming voltage to the Y terminal of the CC 77. Inthe CC 77, the X terminal is forced by feedback to have the same voltageas that of the Y terminal. Also, the current on the X terminal isduplicated into the Z terminal of the CC 77. A current-controlledvoltage source (CCVS) 79 has a positive node and a negative node. TheCCVS 79 converts the current on the Z terminal of the CC 77 into avoltage Vext.

Vext is provided to the display controller and scheduler 64 of FIG. 3A,where the threshold voltage of the driving transistor 73 is extracted.The display controller and scheduler 64 controls the voltage source 78based on the extracted threshold voltage.

FIG. 5A illustrates a further example of a system for thevoltage-extracting, programming, and driving of FIG. 3A, which isemployed with a bottom-emission current-cell pixel circuit 80. The pixelcircuit 80 includes an OLED 81, a storage capacitor 82, a drivingtransistor 83, and switch transistors 84 and 85. The transistors 83, 84and 85 may be n-type TFTs. However, these transistors 83, 84 and 85 maybe p-type transistors.

The driving transistor 83 is connected to a data line 86 through theswitch transistor 85, and is connected to the OLED 81, and also isconnected to the storage capacitor 82. The gate terminal of the drivingtransistor 83 is connected to a voltage supply line VDD through theswitch transistor 84. The gate terminals of the switch transistors 84and 85 are connected to a select line SEL. The pixel circuit 80 isselected by the select line SEL and is driven by DATA on the data line86.

A current conveyor (CC) 87 has X, Y and Z terminals, and is used toextract a current on the data line 86 without loading it. A voltagesource 88 applies a negative programming voltage at the Y terminal ofthe CC 87. In the CC 87, the X terminal is forced by feedback to havethe same voltage as that of the Y terminal. Also, the current on the Xterminal is duplicated into the Z terminal of the CC 87. Acurrent-controlled voltage source (CCVS) 89 has a positive node and anegative node. The CCVS 89 converts the current of the Z terminal of theCC 87 into a voltage Vext.

Vext is provided to the display controller and scheduler 64 of FIG. 3A,where the threshold voltage of the driving transistor 83 is extracted.The display controller and scheduler 64 controls the voltage source 88based on the extracted threshold voltage.

FIG. 6A illustrates a further example of a system for thevoltage-extracting, programming and driving of FIG. 3A, which isemployed with a top-emission current-mirror pixel circuit 90. The pixelcircuit 90 includes an OLED 91, a storage capacitor 92, mirrortransistors 93 and 94, and switch transistors 95 and 96. The transistors93, 94, 95 and 96 may be n-type TFTs. However, these transistors 93, 94,95 and 96 may be p-type transistors.

The mirror transistor 93 is connected to a data line 97 through theswitch transistor 95, and is connected to the storage capacitor 92through the switch transistor 96. The gate terminals of the mirrortransistors 93 and 94 are connected to the storage capacitor 92 and theswitch transistor 96. The mirror transistor 94 is connected to a voltagesupply electrode or line VDD through the OLED 91. The gate terminals ofthe switch transistors 85 and 86 are connected to a select line SEL. Thepixel circuit 90 is selected by the select line SEL and is driven byDATA on the data line 97.

A current conveyor (CC) 98 has X, Y and Z terminals, and is used toextract the current of the data line 97 without loading it. A voltagesource 99 applies a positive programming voltage at the Y terminal ofthe CC 98. In the CC 98, the X terminal is forced by feedback to havethe same voltage as the voltage of the Y terminal. Also, the current onthe X terminal is duplicated into the Z terminal of the CC 98. Acurrent-controlled voltage source (CCVS) 100 has a positive node and anegative node. The CCVS 100 converts a current on the Z terminal of theCC 98 into a voltage Vext.

Vext is provided to the display controller and scheduler 64 of FIG. 3A,where the threshold voltage of the driving transistor 93 is extracted.The display controller and scheduler 64 controls the voltage source 99based on the extracted threshold voltage.

FIG. 7A illustrates a further example of a system for thevoltage-extracting, programming and driving of FIG. 3A, which isemployed with a bottom-emission current-minor pixel circuit 110. Thepixel circuit 110 includes an OLED 111, a storage capacitor 112, mirrortransistors 113 and 116, and switch transistors 114 and 115. Thetransistors 113, 114, 115 and 116 may be n-type TFTs. However, thesetransistors 113, 114, 115 and 116 may be p-type transistors.

The mirror transistor 113 is connected to a data line 117 through theswitch transistor 114, and is connected to the storage capacitor 112through the switch transistor 115. The gate terminals of the mirrortransistors 113 and 116 are connected to the storage capacitor 112 andthe switch transistor 115. The minor transistor 116 is connected to avoltage supply line VDD. The mirror transistors 113, 116 and the storagecapacitor 112 are connected to the OLED 111. The gate terminals of theswitch transistors 114 and 115 are connected to a select line SEL. Thepixel circuit 110 is selected by the select line SEL and is driven byDATA on the data line 117.

A current conveyor (CC) 118 has X, Y and Z terminals, and is used toextract the current of the data line 117 without loading it. A voltagesource 119 applies a positive programming voltage at the Y terminal ofthe CC 118. In the CC 118, the X terminal is forced by feedback to havethe same voltage as the voltage of the Y terminal of the CC 118. Also,the current on the X terminal is duplicated into the Z terminal of theCC 118. A current-controlled voltage source (CCVS) 120 has a positivenode and a negative node. The 120 converts the current on the Z terminalof the CC 118 into a voltage Vext.

Vext is provided to the display controller and scheduler 64 of FIG. 3A,where the threshold voltage of the driving transistor 113 is extracted.The display controller and scheduler 64 controls the voltage source 119based on the extracted threshold voltage.

Referring to FIGS. 3A-7A, using the voltage-extracting technique, timedependent parameters of a pixel (e.g. threshold shift) can be extracted.Thus, the programming voltage can be calibrated with the extractedinformation, resulting in a stable pixel current over time. Since thevoltage of the OLED (i.e. 71 of FIG. 4A, 81 of FIG. 5A, 91 of FIG. 6A,111 of FIG. 7A) affects the current directly, the voltage-extractingdriving technique described above can also be used to extract OLEDdegradation as well as the threshold shift.

The voltage-extracting technique described above can be used with anycurrent-mode pixel circuit, including current-mirror and current-cellpixel circuit architectures, and are applicable to the display array 10of FIG. 2A. A stable current independent of pixel aging under prolongeddisplay operation can be provided using the extracted information. Thus,the display operating lifetime is efficiently improved.

It is noted that the transistors in the pixel circuits of FIGS. 3A-7Amay be fabricated using amorphous silicon, nano/micro crystallinesilicon, poly silicon, organic semiconductors technologies (e.g. organicTFT), NMOS/PMOS technology or CMOS technology (e.g. MOSFET). The pixelcircuits of FIGS. 3A-7A may form AMOLED display arrays.

A further technique for programming, extracting time dependentparameters of a pixel and driving the pixel is described in detail withreference to FIGS. 8A-17A. The technique includes a step-calibrationdriving technique. In the step-calibration driving technique,information on the aging of a pixel (e.g. threshold shift) is extracted.The extracted information will be used to generate a stable pixelcurrent/luminance. Despite using the one-bit extraction technique, theresolution of the extracted aging is defined by display drivers. Also,the dynamic effects are compensated since the pixel aging is extractedunder operating condition, which is similar to the driving cycle.

FIG. 8A illustrates a pixel circuit 160 to which a step-calibrationdriving in accordance with an embodiment of the present invention isapplied. The pixel circuit 160 includes an OLED 161, a storage capacitor162, and a driving transistor 163 and switch transistors 164 and 165.The pixel circuit 160 is a current-programmed, 3-TFT pixel circuit. Aplurality of the pixel circuits 160 may form an AMOLED display.

The transistors 163, 164 and 165 are n-type TFTs. However, thetransistors 163, 164 and 165 may be p-type TFTs. The step-calibrationdriving technique applied to the pixel circuit 160 is also applicable toa pixel circuit having p-type transistors. The transistors 163, 164 and165 may be fabricated using amorphous silicon, nano/micro crystallinesilicon, poly silicon, organic semiconductors technologies (e.g. organicTFT), NMOS/PMOS technology or CMOS technology (e.g. MOSFET).

The gate terminal of the driving transistor 163 is connected to a signalline VDATA through the switch transistor 164, and also connected to thestorage capacitor 162. The source terminal of the driving transistor 163is connected to a common ground. The drain terminal of the drivingtransistor 163 is connected to a monitor line MONITOR through the switchtransistor 165, and also is connected to the cathode electrode of theOLED 161.

The gate terminal of the switch transistor 164 is connected to a selectline SELL. The source terminal of the switch transistor 164 is connectedto the gate terminal of the driving transistor 163, and is connected tothe storage capacitor 162. The drain terminal of the switch transistor164 is connected to VDATA.

The gate terminal of the switch transistor 165 is connected to a selectline SEL2. The source terminal of the switch transistor 165 is connectedto MONITOR. The drain terminal of the switch transistor 165 is connectedto the drain terminal of the driving transistor 163 and the cathodeelectrode of the OLED 161. The anode electrode of the OLED 161 isconnected to a voltage supply electrode or line VDD.

The transistors 163 and 164 and the storage capacitor 162 are connectedat node A3. The transistors 163 and 165 and the OLED 161 are connectedat node B3.

FIG. 9A illustrates an example of a driver and extraction block 170along with the driving transistor 163 of FIG. 8A. In FIG. 9A, each of Rs171 a and Rs 171 b represents the ON resistance of the switchtransistors (e.g. 164, 165 of FIG. 8A). Cs represents the storagecapacitor of the pixel, C_(OLED) represents the OLED capacitance, and CPrepresents the line parasitic capacitance. In FIG. 9A, the OLED ispresented as a capacitance.

A block 173 is used to extract the threshold voltage of the drivingtransistor, during the extraction cycle. The block 173 may be a currentsense amplifier (SA) or a current comparator. In the description, theblock 173 is referred to as “SA block 173”.

If the current of the MONITOR line is higher than a reference current(IREF), the output of the SA block 173 (i.e. Triggers of FIG. 10A, 11A)becomes one. If the current of the MONITOR line is less than thereference current (IREF), the output of the SA block 173 becomes zero.

It is noted that the SA block 173 can be shared between few columnsresult in less overhead. Also, the calibration of the pixel circuit canbe done one at a time, so the extraction circuits can be shared betweenthe all columns.

A data process unit (DPU) block 172 is provided to control theprogramming cycle, contrast, and brightness, to perform the calibrationprocedure and to control the driving cycle. The DPU block 172 implementsextraction algorithm to extract (estimate) the threshold voltage of thedriving transistor based on the output from the SA block 173, andcontrols a driver 174 which is connected to the driving transistor 163.

FIG. 10A illustrates an example of the extraction algorithm implementedby the DPU block 172 of FIG. 9A. The algorithm of FIG. 10A is in a partof the DPU block 172. In FIG. 10A, V_(T)(i, j) represents the extractedthreshold voltage for the pixel (i, j) at the previous extraction cycle,V_(S) represents the resolution of the driver 174, “i” represents a rowof a pixel array and “j” represents a column of a pixel array. Triggerconveys the comparison results of the SA block 173 of FIG. 9A.Less_state 180 determines the situation in which the actual V_(T) of thepixel is less than the predicted V_(T)(V_(TM)), Equal_state 181determines the situation in which the predicted V_(T)(V_(TM)) and theactual V_(T) of the pixel are equal, and Great state 182 determines thesituation in which the actual V_(T) of the pixel is greater than thepredicted V_(T) (V_(TM)).

The DPU block 172 of FIG. 9A determines an intermediate thresholdvoltage V_(TM) as follows:

(A1) When s(i, j)Less_state (180), the actual threshold voltage is lessthan V_(T)(i, j), V_(TM) is set to (V_(T)(i, j)−V_(S)).

(A2) When s(i, j)=Equal_state (181), the actual threshold voltage isequal to VT(i, j), VTM is set to VT (i, j).

(A3) When s(i, j)=Greater_state (182), the actual threshold voltage isgreater than V_(T)(i, j), V_(TM) is set to (V_(T)(i, j)±V_(S)).

where s(i, j) represents the previous state of the pixel (i, j) storedin a calibration memory (e.g. 208 of FIG. 16A).

FIG. 11A illustrates a further example of the extraction algorithmimplemented by the DPU block 172 of FIG. 9A. The algorithm of FIG. 11Ais in a part of the DPU block 172 of FIG. 9A. In FIG. 11A, V_(T)(i, j)represents the extracted threshold voltage for the pixel (i, j) at theprevious extraction cycle, V_(S) represents the resolution of the driver174, “i” represents a row of a pixel array and “j” represents a columnof a pixel array. Trigger conveys the comparison results of the SA block173.

Further, in FIG. 11A, Vres represents the step that will beadded/subtracted to the predicted V_(T) (V_(TM)) in order achieve theactual V_(T) of the pixel, A represents the reduction gain of aprediction step, and K represents the increase gain of the predictionstep.

The operation of FIG. 11A is the same as that of FIG. 10A, except thatit has gain extra states L2 and G2 for rapid extraction of abruptchanges. In the gain states, the step size is increased to follow thechanges more rapidly. L1 and G1 are the transition states which definethe V_(T) change is abrupt or normal.

FIG. 12A illustrates an example of waveforms applied to the pixelcircuit 160 of FIG. 8A. In FIG. 12A, V_(call)=V_(B)+V_(TM), andV_(DR)=V_(P)+V_(T)(i, j)+V_(REF), where V_(B) represents the biasvoltage during the extraction cycle, V_(TM) is defined based on thealgorithm shown in FIG. 10A or 11A, V_(P) represents a programmingvoltage, V_(T)(i, j) represents the extracted threshold voltage at theprevious extraction cycle, V_(REF) represents the source voltage of thedriving transistor during the programming cycle.

Referring to FIGS. 8A-12A, the operation of the pixel circuit 160includes operating cycles X51, X52, X53, and X54. In FIG. 12A, anextraction cycle is separated from a programming cycle. The extractioncycle includes X51 and X52, and the programming cycle includes X53. X54is a driving cycle. At the end of the programming cycle, node A3 ischarged to (V_(P)+V_(T)) where V_(P) is a programming voltage and V_(T)is the threshold voltage of the driving transistor 163.

In the first operating cycle X51: SEL1 and SEL 2 are high. Node A3 ischarged to V_(cal), and node B3 is charged to V_(REF). V_(cal) isV_(B)±V_(TM) in which V_(B) is a bias voltage, and V_(TM) the predictedV_(T), and V_(REF) should be larger than V_(DD)−V_(OLED0) whereV_(OLED0) is the ON voltage of the OLED 161.

In the second operating cycle X52: SEL1 goes to zero. The gate-sourcevoltage of the driving transistor 163 is given by:VGS=V _(B) =V _(TM) +ΔV _(B) +ΔV _(TM) −V _(T2) −ΔV _(H)where VGS represents the gate-source voltage of the driving transistor163, ΔV_(B), ΔV_(TM), ΔV_(T2) and ΔV_(H) are the dynamic effectsdepending on V_(B), V_(TM), V_(T2) and V_(H), respectively. V_(T2)represents the threshold voltage of the switch transistor 164, and V_(H)represents the change in the voltage of SEL1 at the beginning of secondoperating cycle X52 when it goes to zero.

The SA block 173 is tuned to sense the current larger than β(V_(B))², sothat the gate-source voltage of the driving transistor 163 is largerthan (V_(B)+V_(T)), where β is the gain parameter in the I-Vcharacteristic of the driving transistor 163.

As a result, after few iterations, V_(TM) and the extracted thresholdvoltage V_(T)(i, j) for the pixel (i, j) converge to:

V_(TM) = V_(T) − γ ⋅ (V_(B) + V_(T) + V_(T 2) − V_(H))$\gamma = \frac{G_{g\; 2}\text{/}\left( {2 \cdot C_{S}} \right)}{1 + {C_{g\; 2}\text{/}\left( {2 \cdot C_{S}} \right)}}$where C_(g2) represents the gate capacitance of the switch transistor164.

In the third operating cycle X53: SEL1 is high. VDATA goes to V_(DR).Node A3 is charged to [V_(P)+V_(T)(i, j)−γ(V_(P)−V_(B))].

In the fourth operating cycle X54: SEL1 and SEL2 go to zero. Consideringthe dynamic effects, the gate-source voltage of the driving transistor163 can be written as:VGS=V _(P) +V _(T)

Therefore, the pixel current becomes independent of the static anddynamic effects of the threshold voltage shift.

In FIG. 12A, the extraction cycle and the programming cycle are shown asseparated cycles. However, the extraction cycle and the programmingcycle may be merged as shown in FIG. 13A. FIG. 13A illustrates a furtherexample of waveforms applied to the pixel circuit 160 of FIG. 8A.

Referring to FIGS. 8A-11A and 13A, the operation of the pixel circuit160 includes operating cycles X61, X62 and X63. Programming andextraction cycles are merged into the operating cycles X61 and X62. Theoperating cycle X63 is a driving cycle.

During the programming cycle, the pixel current is compared with thedesired current, and the threshold voltage of the driving transistor isextracted with the algorithm of FIG. 10A or 11A. The pixel circuit 160is programmed with V_(DR)=VP+V_(T) (i, j)+V_(REF) during the operatingcycle X61. Then the pixel current is monitored through the MONITOR line,and is compared with the desired current. Based on the comparison resultand using the extraction algorithm of FIG. 10A or 11A, the thresholdvoltage V_(T) (i, j) is updated.

In FIG. 8A, two select lines SEL1 and SEL2 are shown. However, a signalselect line (e.g. SEL1) can be used as a common select line to operatethe switch transistors 164 and 165. When using the common select line,SEL1 of FIG. 12A stays at high in the second operating cycle X52, andthe VGS remains at (V_(B)+V_(TM)). Therefore, the dynamic effects arenot detected.

The step-calibration driving technique described above is applicable tothe pixel circuit 190 of FIG. 14A. The pixel circuit 190 includes anOLED 191, a storage capacitor 192, and a driving transistor 193 andswitch transistors 194 and 195. The pixel circuit 190 is acurrent-programmed, 3-TFT pixel circuit. A plurality of the pixelcircuits 190 may form an AMOLED display.

The transistors 193, 194 and 195 are n-type TFTs. However, thetransistors 193, 194 and 195 may be p-type TFTs. The step-calibrationdriving technique applied to the pixel circuit 190 is also applicable toa pixel circuit having p-type transistors. The transistors 193, 194 and195 may be fabricated using amorphous silicon, nano/micro crystallinesilicon, poly silicon, organic semiconductors technologies (e.g. organicTFT), NMOS/PMOS technology or CMOS technology (e.g. MOSFET).

The gate terminal of the driving transistor 193 is connected to a signalline VDATA through the switch transistor 194, and also connected to thestorage capacitor 192. The source terminal of the driving transistor 193is connected to the anode electrode of the OLED 191, and is connected toa monitor line MONITOR through the switch transistor 195. The drainterminal of the driving transistor 193 is connected to a voltage supplyline VDD. The gate terminals of the transistors 194 and 195 areconnected to select lines SEL1 and SEL2, respectively.

The transistors 193 and 194 and the storage capacitor 192 are connectedat node A4. The transistor 195, the OLED 191 and the storage capacitor192 are connected at node B4.

The structure of the pixel circuit 190 is similar to that of FIG. 8A,except that the OLED 191 is at the source terminal of the drivingtransistor 193. The operation of the pixel circuit 190 is the same asthat of FIG. 12A or 13A.

Since the source terminal of the drive TFT 193 is forced to VREF duringthe extraction cycle (X51 and X52 or X62), the extracted data isindependent of the ground bouncing. Also, during the programming cycle(X53 or X61), the source terminal of the drive TFT is forced to VREF,the gate-source voltage of the drive TFT becomes independent of theground bouncing. As a result of these conditions, the pixel current isindependent of ground bouncing.

FIG. 15A illustrates the results of simulation for the step-calibrationdriving technique. In FIG. 15A, “Case I” represents an operation of FIG.8A where SEL1 goes to zero in the second operating cycle (X52 of FIG.12A); “Case II” represents an operation of FIG. 8A where SEL1 stays athigh in the second operating cycle.

In FIG. 15A, ΔV_(TR) is the minimum detectable shift in the thresholdvoltage of the driving transistor (e.g. 163 of FIG. 8A), ΔV_(T2R) is theminimum detectable shift in the threshold voltage of the switchtransistor (e.g. 164 of FIG. 8A), and In is the pixel current of thepixel during the driving cycle.

The pixel current of Case II is smaller than that of Case I for a givenprogramming voltage due to the dynamic effects of the threshold voltageshift. Also, the pixel current of Case II increases as the thresholdvoltage of the driving transistor increases (a), and decreases as thethreshold voltage of the switch transistor decreases (b). However, thepixel current of Case I is stable. The maximum error induced in thepixel current is less than %0.5 for any shift in the threshold voltageof the driving and switch TFTs. It is obvious that ΔV_(T2R) is largerthan ΔV_(TR) because the effect of a shift in VT on the pixel current isdominant. These two parameters are controlled by the resolution (V_(S))of the driver (e.g. 174 of FIG. 9A), and the SNR of the SA block (e.g.193 of FIG. 9A). Since a shift smaller than ΔV_(TR) cannot be detected,and also the time constant of threshold-shift is large, the extractioncycles (e.g. X51, X52 of FIG. 12A) can be done after a long timeinterval consisting of several frames, leading to lower powerconsumption. Also, the major operating cycles become the otherprogramming cycle (e.g. X53 of FIG. 12A) and the driving cycle (e.g. X54of FIG. 12A). As a result, the programming time reduces significantly,providing for high-resolution, large-area AMOLED displays where ahigh-speed programming is prerequisite.

FIG. 16A illustrates an example of a system architecture for thestep-calibration driving with a display array 200. The display array 200includes a plurality of the pixel circuits (e.g. 160 of FIG. 8A or 190of FIG. 14A).

A gate driver 202 for selecting the pixel circuits, a drivers/SAs block204, and a data process and calibration unit block 206 are provided tothe display array 200. The drivers/SAs block 204 includes the driver 174and the SA block 173 of FIG. 9A. The data process and calibration unitblock 206 includes the DPU block 172 of FIG. 9A. “Calibration” in FIG.16A includes the calibration data from a calibration memory 208, and mayinclude some user defined constants for setting up calibration dataprocessing. The contrast and the brightness inputs are used to adjustthe contrast and the brightness of the panel by the user. Also,gamma-correction data is defined based on the OLED characteristic andhuman eye. The gamma-correction input is used to adjust the pixelluminance for human eyes.

The calibration memory 208 stores the extracted threshold voltageV_(T)(i, j) and the state s(i, j) of each pixel. A memory 210 stores theother required data for the normal operation of a display includinggamma correction, resolution, contrast, and etc. The DPU block performsthe normal tasks assigned to a controller and scheduler in a display.Besides, the algorithm of FIG. 10A or 11A is added to it to perform thecalibration.

FIG. 17A illustrates an example of waveforms applied to the systemarchitecture of FIG. 16A. In FIG. 17A, each of ROW[1], ROW[2], andROW[3] represents a row of the display array 200, “E” represents anextraction operation, “P” represents a programming operation and “D”represents a driving operation. It is noted that the extraction cycles(E) are not required to be done for all the frame cycle. Therefore,after a long time interval (extraction interval), the extraction isrepeated for a pixel.

As shown in FIG. 17A, only one extraction procedure occurs during aframe time. Also, the VT extraction of the pixel circuits at the samerow is preformed at the same time.

Therefore, the maximum time required to refresh a frame is:τ_(F) =n·τ _(P)+τ_(E)where τ_(F). represents the frame time, τ_(P) represents the timerequired to write the pixel data into the storage capacitor (e.g. 162 ofFIG. 8A), τ_(E) represents the extraction time, and n represents thenumber of row in the display array (e.g. 200 of FIG. 16A).

Assuming τ_(E)=m·τ_(P), the frame time τ_(F) can be written as:τ_(F)=(n+m)·τ_(P)where m represents the timing required for the extraction cycles in thescale of programming cycle timing (τP).

For example, for a Quarter Video Graphics Array (QVGA) display (240×320)with frame rate of 60 Hz, if m=10, the programming time of each row is6611 s, and the extraction time is 0.66 ms.

It is noted that the step-calibration driving technique described aboveis applicable to any current-programmed pixel circuit other than thoseof FIGS. 8A and 14A.

Using the step-calibration driving technique, the time dependentparameter(s) of a pixel, such as threshold shift, is extracted. Then,the programming-voltage is calibrated with the extracted information,resulting in a stable pixel current over time. Further, a stable currentindependent of the pixel aging under prolonged display operation can beis provided to the pixel circuit, which efficiently improves the displayoperating lifetime.

A technique for programming, extracting time dependent parameters of apixel and driving the pixel in accordance with a further embodiment ofthe present invention is described in detail. The technique includesextracting information on the aging of a pixel (e.g. OLED luminance) bymonitoring OLED voltage or OLED current, and generating luminance. Theprogramming voltage is calibrated with the extracted information,resulting in stable brightness over time.

Since the OLED voltage/current has been reported to be correlated withthe brightness degradation in the OLED (e.g. 161 of FIG. 8A, 191 of FIG.14A), the programming voltage can be modified by the OLEDvoltage/current to provide a constant brightness.

For example, during the driving cycle, the voltage/current of the OLED(161 of FIG. 8A or 191 of FIG. 14A) is extracted while SEL2 is high.Since the OLED voltage or current has been reported to be correlatedwith the brightness degradation in the OLED, the programming voltage canbe modified by the OLED voltage to provide a constant brightness.

FIG. 18A illustrates an example of waveforms for the voltage/currentextraction. The waveforms of FIG. 18A are applicable to the pixelcircuit 160 of FIG. 8A and the pixel circuit 190 of FIG. 14A to extractOLED voltage/current. The operation of FIG. 18A includes operatingcycles X71, X72 and X73. The operating cycles X71 and X72 are an OLEDextraction cycle. The operating cycle X73 is one of the operating cyclesshown in FIGS. 12A and 13.

During the first operating cycle X71, SEL1 and SEL2 are high, and VDATAis zero. The gate-source voltage of the driving transistor (e.g. 163 ofFIG. 8A) becomes zero. A current or voltage is applied to the OLED (161of FIG. 8A) through the MONITOR line.

During the second operating cycle X72, SEL2 is high and SEL1 is low. TheOLED voltage or current is extracted through the MONITOR line using thealgorithm presented in FIG. 10A or 11A. This waveform can be combinedwith any other driving waveform.

In the above description, the algorithm of FIGS. 10A and 11A is used topredict the aging data, i.e. V_(T) shift, based on the comparisonresults (current with current or voltage with voltage). However, thealgorithm of FIGS. 10A and 11A is applicable to predict the shift in theOLED voltage V_(OLED) by replacing V_(T) with the V_(OLED) and thecomparison result of OLED current/voltage with a referencecurrent/voltage. In the description above, the system architecture shownin FIG. 9A is used to compensate for the threshold shift. However, it isunderstood that the OLED data is also extracted when the architecture ofFIG. 9A, i.e. DPU 172, block 173, driver 174, is used. This data can beused to compensate for the OLED shift.

The operating cycle X73 can be any operating cycle including theprogramming cycle. This depends on the status of the panel after OLEDextraction. If it is during the operation, then X73 is the programmingcycle of the waveforms in FIGS. 12A and 13A. The OLED voltage can beextracted during the driving cycle X55/X63 of FIG. 12A/13A. During thedriving cycle X55/X63, the SEL2 of FIG. 8A or 14A goes to a highvoltage, and so the voltage of the OLED can be read back through theMONITOR for a specific pixel current.

FIG. 19A illustrates a further example of waveforms for thevoltage/current extraction. FIG. 20A illustrates a pixel circuit 220 towhich the voltage/current extraction of FIG. 19A is applied.

Referring to FIG. 20A, the pixel circuit 220 includes an OLED 221, astorage capacitor 222, and a driving transistor 223 and switchtransistors 224 and 225. A plurality of the pixel circuits 220 may forman AMOLED display.

The transistors 223, 224 and 225 are n-type TFTs. However, thetransistors 223, 224 and 225 may be p-type TFTs. The voltage/currentextraction technique applied to the pixel circuit 220 is also applicableto a pixel circuit having p-type transistors. The transistors 223, 224and 225 may be fabricated using amorphous silicon, nano/microcrystalline silicon, poly silicon, organic semiconductors technologies(e.g. organic TFT), NMOS/PMOS technology or CMOS technology (e.g.MOSFET).

The gate terminal of the driving transistor 223 is connected to thesource terminal of the switch transistor 224, and also connected to thestorage capacitor 222. The one terminal of the driving transistor 223 isconnected to a common ground. The other terminal of the drivingtransistor 223 is connected to a monitor and data line MONITOR/DATAthrough the switch transistor 235, and is also connected to the cathodeelectrode of the OLED 221.

The gate terminal of the switch transistor 224 is connected to a selectline SELL. The one terminal of the switch transistor 224 is connected tothe gate terminal of the driving transistor 223, and is connected to thestorage capacitor 222. The other terminal of the switch transistor 224is connected to the cathode electrode of the OLED 221.

The gate terminal of the switch transistor 225 is connected to a selectline SEL2. The one terminal of the switch transistor 225 is connected toMONITOR/DATA. The other terminal of the switch transistor 225 isconnected to the driving transistor 223 and the cathode electrode of theOLED 221. The anode electrode of the OLED 221 is connected to a voltagesupply electrode or line VDD.

The transistors 223 and 224 and the storage capacitor 222 are connectedat node A5. The transistors 223 and 225 and the OLED 221 are connectedat node B5.

The pixel circuit 220 is similar to the pixel circuit 160 of FIG. 8A.However, in the pixel circuit 220, the MONITOR/DATA line is used formonitoring and programming purpose.

Referring to FIGS. 19A-20A, the operation of the pixel circuit 220includes operating cycles X81, X82 and X83.

During the first operating cycle X81, SEL1 and SEL2 are high andMONITOR/DATA is zero. The gate-source voltage of the driving transistor(223 of FIG. 20A) becomes zero.

During the second operating cycle X82, a current or voltage is appliedto the OLED through the MONITOR/DATA line, and its voltage or current isextracted. As described above, the shift in the OLED voltage isextracted using the algorithm presented in FIG. 10A or 11A based on themonitored voltage or current. This waveform can be combined with anydriving waveform.

The operating cycle X83 can be any operating cycle including theprogramming cycle. This depends on the status of the panel after OLEDextraction.

The OLED voltage/current can be extracted during the driving cycle ofthe pixel circuit 220 of FIG. 20A after it is programmed for a constantcurrent using any driving technique. During the driving cycle the SEL2goes to a high voltage, and so the voltage of the OLED can be read backthrough the MONITOR/DATA line for a specific pixel current.

FIG. 21A illustrates a further example of waveforms for thevoltage/current extraction technique. FIG. 22A illustrates a pixelcircuit 230 to which the voltage/current extraction of FIG. 21A isapplied. The waveforms of FIG. 21A is also applicable to the pixelcircuit 160 of FIG. 8A to extract OLED voltage/current.

Referring to FIG. 22A, the pixel circuit 230 includes an OLED 231, astorage capacitor 232, and a driving transistor 233 and switchtransistors 234 and 235. A plurality of the pixel circuits 230 may forman AMOLED display.

The transistors 233, 234 and 235 are n-type TFTs. However, thetransistors 233, 234 and 235 may be p-type TFTs. The voltage/currentextraction technique applied to the pixel circuit 230 is also applicableto a pixel circuit having p-type transistors. The transistors 233, 234and 235 may be fabricated using amorphous silicon, nano/microcrystalline silicon, poly silicon, organic semiconductors technologies(e.g. organic TFT), NMOS/PMOS technology or CMOS technology (e.g.MOSFET).

The gate terminal of the driving transistor 233 is connected to thesource terminal of the switch transistor 234, and also connected to thestorage capacitor 232. The one terminal of the driving transistor 233 isconnected to a voltage supply line VDD. The other terminal of thedriving transistor 233 is connected to a monitor and data lineMONITOR/DATA through the switch transistor 235, and is also connected tothe anode electrode of the OLED 231.

The gate terminal of the switch transistor 234 is connected to a selectline SELL. The one terminal of the switch transistor 234 is connected tothe gate terminal of the driving transistor 233, and is connected to thestorage capacitor 232. The other terminal of the switch transistor 234is connected to VDD.

The gate terminal of the switch transistor 225 is connected to a selectline SEL2. The one terminal of the switch transistor 235 is connected toMONITOR/DATA. The other terminal of the switch transistor 235 isconnected to the driving transistor 233 and the anode electrode of theOLED 231. The anode electrode of the OLED 231 is connected to VDD.

The transistors 233 and 234 and the storage capacitor 232 are connectedat node A6. The transistors 233 and 235 and the OLED 231 are connectedat node B5.

The pixel circuit 230 is similar to the pixel circuit 190 of FIG. 14A.However, in the pixel circuit 230, the MONITOR/DATA line is used formonitoring and programming purpose.

Referring to FIGS. 21A-22A, the operation of FIG. 22A includes operatingcycles X91, X92 and X93.

During the first operating cycle X91, SEL1 and SEL2 are high and VDDgoes to zero. The gate-source voltage of the driving transistor (e.g.233 of FIG. 21A) becomes zero.

During the second operating cycle X92, a current (voltage) is applied tothe OLED (e.g. 231 of FIG. 21A) through the MONITOR/DATA line, and itsvoltage (current) is extracted. As described above, the shift in theOLED voltage is extracted using the algorithm presented in FIG. 10A or11A based on the monitored voltage or current. This waveform can becombined with any other driving waveform.

The operating cycle X93 can be any operating cycle including theprogramming cycle. This depends on the status of the panel after OLEDextraction.

The OLED voltage can be extracted during the driving cycle of the pixelcircuit 230 of FIG. 21A after it is programmed for a constant currentusing any driving technique. During the driving cycle the SEL2 goes to ahigh voltage, and so the voltage of the OLED can be read back throughthe MONITOR/DATA line for a specific pixel current.

As reported, the OLED characteristics improve under negative biasstress. As a result, a negative bias related to the stress history ofthe pixel, extracted from the OLED voltage/current, can be applied tothe OLED during the time in which the display is not operating. Thismethod can be used for any pixel circuit presented herein.

Using the OLED voltage/current extraction technique, a pixel circuit canprovide stable brightness that is independent of pixel aging underprolonged display operation, to efficiently improve the displayoperating lifetime.

A technique for reducing the unwanted emission in a display array havinga light emitting device in accordance with an embodiment of the presentinvention is described in detail. This technique includes removing OLEDfrom a programming path during a programming cycle. This technique canbe adopted in hybrid driving technique to extract information on theprecise again of a pixel, e.g. the actual threshold voltageshift/mismatch of the driving transistor. The light emitting device isturned off during the programming/calibration cycle so that it preventsthe unwanted emission and effect of the light emitting device on thepixel aging. This technique can be applied to any current mirror pixelcircuit fabricated in any technology including poly silicon, amorphoussilicon, crystalline silicon, and organic materials.

FIG. 23A illustrates a mirror based pixel circuit 250 to which atechnique for removing OLED from a programming path during a programmingcycle is applied. The pixel circuit 250 includes an OLED 251, a storagecapacitor 252, a programming transistor 253, a driving transistor 254,and switch transistors 255 and 256. The gate terminals of thetransistors 253 and 254 are connected to IDATA through the switchtransistors 255 and 256.

The transistors 253, 254, 255 and 256 are n-type TFTs. However, thetransistors 253, 254, 255 and 256 may be p-type TFTs. The OLED removingtechnique applied to the pixel circuit 250 is also applicable to a pixelcircuit having p-type transistors. The transistors 253, 254, 255 and 256may be fabricated using amorphous silicon, nano/micro crystallinesilicon, poly silicon, organic semiconductors technologies (e.g. organicTFT), NMOS/PMOS technology or CMOS technology (e.g. MOSFET).

The transistors 253, 254 and 256 and the storage capacitor 252 areconnected at node A10. The transistors 253 and 254, the OLED 251 and thestorage capacitor 252 are connected at node B10.

In the conventional current programming, SEL goes high, and aprogramming current (IP) is applied to IDATA. Considering that the widthof the mirror transistor 253 is “m” times larger than the width of themirror transistor 254, the current flowing through the OLED 251 duringthe programming cycle is (m+1)IP. When “m” is large to gain significantspeed improvement, the unwanted emission may become considerable.

By contrast, according to the OLED removing technique, VDD is broughtinto a lower voltage. This ensures the OLED 251 to be removed from aprogramming path as shown in FIG. 24A.

During a programming cycle, SEL is high and VDD goes to a referencevoltage (Vref) in which the OLED 251 is reversely biased. Therefore, theOLED 251 is removed from the current path during the programming cycle.

During the programming cycle, the pixel circuit 250 may be programmedwith scaled current through IDATA without experiencing unwantedemission.

During the programming cycle, the pixel circuit 250 may be programmedwith current and using one of the techniques describe above. The voltageof the IDATA line is read back to extract the threshold voltage of themirror transistor 253 which is the same as threshold voltage of thedriving transistor 254.

Also, during the programming cycle, the pixel circuit 250 may beprogrammed with voltage through the IDATA line, using one of thetechniques describe above. The current of the IDATA line is read back toextract the threshold voltage of the mirror transistor 253 which is thesame as threshold voltage of the driving transistor 254.

The reference voltage Vref is chosen so that the voltage at node B10becomes smaller than the ON voltage of the OLED 251. As a result, theOLED 251 turns off and the unwanted emission is zero. The voltage of theIDATA line includesV _(P) +V _(T) +ΔVT  (3)where V_(P) includes the drain-source voltage of the driving transistor254 and the gate-source voltage of the transistor 253, V_(T) is thethreshold voltage of the transistor 253 (254), and ΔV_(T) is the V_(T)shift/mismatch.

At the end of the programming cycle, VDD goes to its original value, andso voltage at node B10 goes to the OLED voltage VOLED. At the drivingcycle, SEL is low. The gate voltage of the transistor 254/253 is fixedand stored in the storage capacitor 252, since the switch transistors255 and 256 are off. Therefore, the pixel current during the drivingcycle becomes independent of the threshold voltage V_(T).

The OLED removing technique can be adopted in hybrid driving techniqueto extract the V_(T)-shift or V_(T)-mismatch. From (3), if the pixel isprogrammed with the current, the only variant parameter in the voltageof the DATA line is the V_(T) shift/mismatch (ΔV_(T)). Therefore, ΔV_(T)can be extracted and the programming data can be calibrated with ΔV_(T).

FIG. 25A illustrates an example of a system architecture forimplementing the OLED removing technique. A display array 260 includes aplurality of pixel circuits, e.g. pixel circuit 250 of FIG. 26A. Adisplay controller and scheduler 262 controls and schedules theoperation of the display array 260. A driver 264 provides operationvoltages to the pixel circuit. The driver provides the operationvoltage(s) to the pixel circuit based on instructions/commands from thedisplay controller and scheduler 262 such that the OLED is removed froma programming path of the pixel circuit, as described above.

The controller and scheduler 262 may include functionality of thedisplay controller and scheduler 64 of FIG. 3A, or may includefunctionality of the data process and calibration unit 206 of FIG. 16A.The system of FIG. 25A may have any of these functionalities, thecalibration-scheduling described above, the voltage/current extractiondescribed above, or combinations thereof.

The simulation result for the voltage on IDATA line for different V_(T)is illustrated in FIG. 26A. Referring to FIGS. 23A-26A, the voltage ofthe IDATA line includes the shift in the threshold voltage of thetransistors 253 and 254. The programming current is 1 μA.

The unwanted emission is reduced significantly resulting in a higherresolution. Also, individual extraction of circuit aging and lightemitting device aging become possible, leading in a more accuratecalibration.

It is noted that each of the transistors shown in FIGS. 4A-8A, 14A, 20A,21A, 23A and 24A can be replaced with a p-type transistor using theconcept of complementary circuits.

All citations are hereby incorporated by reference.

[Start of 13USP1]

Embodiments of the present invention are described using an AMOLEDdisplay which includes a pixel circuit having TFTs and an OLED. However,the transistors in the pixel circuit may be fabricated using amorphoussilicon, nano/micro crystalline silicon, poly silicon, organicsemiconductors technologies (e.g. organic TFT), NMOS technology, CMOStechnology (e.g. MOSFET), or combinations thereof. The transistors maybe a p-type transistor or n-type transistor. The pixel circuit mayinclude a light emitting device other than OLED. In the descriptionbelow, “pixel” and “pixel circuit” may be used interchangeably.

FIG. 3B illustrates the operation of a light emitting display system 100to which a compensation scheme in accordance with an embodiment of thepresent invention is applied. A video source 102 contains luminance datafor each pixel and sends the luminance data in the form of digital data104 to a digital data processor 106. The digital data processor 106 mayperform some data manipulation functions, such as scaling the resolutionor changing the color of the display. The digital data processor 106sends digital data 108 to a data driver IC 110. The data driver IC 110converts that digital data 108 into an analog voltage or current 112.The analog voltage or current 112 is applied to a pixel circuit 114. Thepixel circuit 114 includes TFTs and an OLED. The pixel circuit 114outputs a visible light 126 based on the analog voltage or current 112.

In FIG. 3B, one pixel circuit is shown as an example. However, the lightemitting display system 100 includes a plurality of pixel circuits. Thevideo source 102 may be similar to the video source 12 of FIGS. 1B and2B. The data driver IC 110 may be similar to the data driver IC 20 ofFIGS. 1B and 2B.

A compensation functions module 130 is provided to the display. Thecompensation functions module 130 includes a module 134 for implementingan algorithm (referred to as TFT-to-pixel circuit conversion algorithm)on measurement 132 from the pixel circuit 114 (referred to asdegradation data, measured degradation data, measured TFT degradationdata, or measured TFT and OLED degradation data), and outputs calculatedpixel circuit degradation data 136. It is noted that in the descriptionbelow, “TFT-to-pixel circuit conversion algorithm module” and“TFT-to-pixel circuit conversion algorithm” may be used interchangeably.

The degradation data 132 is electrical data which represents how much apart of the pixel circuit 114 has been degraded. The data measured fromthe pixel circuit 114 may represent, for example, one or morecharacteristics of a part of the pixel circuit 114.

The degradation data 132 is measured from, for example, one or morethin-film-transistors (TFTs), an organic light emitting diode (OLED)device, or a combination thereof. It is noted that the transistors ofthe pixel circuit 114 are not limited to TFTs, and the light emittingdevice of the pixel circuit 114 is not limited to an OLED. The measureddegradation data 132 may be digital or analog data. The system 100provides compensation data based on measurement from a part of the pixelcircuit (e.g. TFT) to compensate for non-uniformities in the display.The non-uniformities may include brightness non-uniformity, colornon-uniformity, or a combination thereof. Factors for causing suchnon-uniformities may include, but are not limited to, process orconstruction inequalities in the display, aging of pixels, etc.

The degradation data 132 may be measured at a regular timing or adynamically regulated timing. The calculated pixel circuit degradationdata 136 may be compensation data to correct non-uniformities in thedisplay. The calculated pixel circuit degradation data 136 may includeany parameters to produce the compensation data. The compensation datamay be used at a regular timing (e.g. each frame, regular interval,etc.) or dynamically regulated timing. The measured data, compensationdata, or a combination thereof may be stored in a memory (e.g. 142 ofFIG. 8B).

The TFT-to-pixel circuit conversion algorithm module 134 or thecombination of the TFT-to-pixel circuit conversion algorithm module 134and the digital data processor 106 estimates the degradation of theentire pixel circuit based on the measured degradation data 132. Basedon this estimation, the entire degradation of the pixel circuit 114 iscompensated by adjusting, at the digital data processor 106, theluminance data (digital data 104) applied to a certain pixel circuit(s).

The system 100 may modify or adjust luminance data 104 applied to adegraded pixel circuit or non-degraded pixel circuit. For example, if aconstant value of visible light 126 is desired, the digital dataprocessor 106 increases the luminance data for a pixel that is highlydegraded, thereby compensating for the degradation.

In FIG. 3B, the TFT-to-pixel circuit conversion algorithm module 134 isprovided separately from the digital data processor 106. However, theTFT-to-pixel circuit conversion algorithm module 134 may be integratedinto the digital data processor 106.

FIG. 4B illustrates an example of the system 100 of FIG. 3B. The pixelcircuit 114 of FIG. 4B includes TFTs 116 and OLED 120. The analogvoltage or current 112 is provided to the TFTs 116. The TFTs 116 convertthat voltage or current 112 into another current 118 which flows throughthe OLED 120. The OLED 120 converts the current 118 into the visiblelight 126. The OLED 120 has an OLED voltage 122, which is the voltagedrop across the OLED. The OLED 120 also has an efficiency 134, which isa ratio of the amount of light emitted to the current through the OLED120.

The system 100 of FIG. 4B measures the degradation of the TFTs only. Thedegradation of the TFTs 116 and the OLED 120 are usage-dependent, andthe TFTs 116 and the OLED 120 are always linked in the pixel circuit114. Whenever the TFT 116 is stressed, the OLED 120 is also stressed.Therefore, there is a predictable relationship between the degradationof the TFTs 116, and the degradation of the pixel circuit 114 as awhole. The TFT-to-pixel circuit conversion algorithm module 134 or thecombination of the TFT-to-pixel circuit conversion algorithm module 134and the digital data processor 106 estimates the degradation of theentire pixel circuit based on the TFT degradation only. An embodiment ofthe present invention may also be applied to systems that monitor bothTFT and OLED degradation independently.

The pixel circuit 114 has a component that can be measured. Themeasurement obtained from the pixel circuit 114 is in some way relatedto the pixel circuit's degradation.

FIG. 5B illustrates an example of the pixel circuit 114 of FIG. 4B. Thepixel circuit 114 of FIG. 5B is a 4-T pixel circuit. The pixel circuit114A includes a switching circuit having TFTs 150 and 152, a referenceTFT 154, a dive TFT 156, a capacitor 158, and an OLED 160.

The gate of the switch TFT 150 and the gate of the feedback TFT 152 areconnected to a select line Vsel. The first terminal of the switch TFT154 and the first terminal of the feedback TFT 152 are connected to adata line Idata. The second terminal of the switch TFT 150 is connectedto the gate of the reference TFT 154 and the gate of the drive TFT 156.The second terminal of the feedback TFT 152 is connected to the firstterminal of the reference TFT 154. The capacitor 158 is connectedbetween the gate of the drive TFT 156 and ground. The OLED 160 isconnected between voltage supply Vdd and the drive TFT 156. The OLED 160may also be connected between drive TFT 156 and ground in other systems(i.e. drain-connected format).

When programming the pixel circuit 114A, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 150 and charges the capacitor 158. As thecapacitor voltage rises, the TFT 154 begins to turn on and Idata startsto flow through the TFTs 152 and 154 to ground. The capacitor voltagestabilizes at the point when all of Idata flows through the TFTs 152 and154. The current flowing through the TFT 154 is mirrored in the driveTFT 156.

In the pixel circuit 114A, by setting Vsel to high and putting a voltageon Idata, the current flowing into the Idata node can be measured.Alternately, by setting Vsel to high and putting a current on Idata, thevoltage at the Idata node can be measured. As the TFTs degrade, themeasured voltage (or current) will change, allowing a measure of thedegradation to be recorded. In this pixel circuit, the analogvoltage/current 112 shown in FIG. 4B is connected to the Idata node. Themeasurement of the voltage or current can occur anywhere along theconnection between the data diver IC 110 and the TFTs 116.

In FIG. 4B, the TFT-to-pixel circuit conversion algorithm is applied tothe measurement 132 from the TFTs 116. However, current/voltageinformation read from various places other than TFTs 116 may be usable.For example, the OLED voltage 122 may be included with the measured TFTdegradation data 132.

FIG. 6B illustrates a further example of the system 100 of FIG. 3B. Thesystem 100 of FIG. 6B measures the OLED voltage 122. Thus, the measureddata 132 is related to the TFT 116 and OLED 120 degradation (“measuredTFT and OLED voltage degradation data 132A” in FIG. 6B). Thecompensation functions module 130 of FIG. 6B implements the TFT-to-pixelcircuit conversion algorithm 134 on the signal related to both the TFTdegradation and OLED degradation. The TFT-to-pixel circuit conversionalgorithm module 134 or the combination of the TFT-to-pixel circuitconversion algorithm module 134 and the digital data processor 106estimates the degradation of the entire pixel circuit based on the TFTdegradation and the OLED degradation. The TFT degradation and OLEDdegradation may be measured separately and independently.

FIG. 7B illustrates an example of the pixel circuit 114 of FIG. 6B. Thepixel circuit 114B of FIG. 7B is a 4-T pixel circuit. The pixel circuit114B includes a switching circuit having TFTs 170 and 172, a referenceTFT 174, a drive TFT 176, a capacitor 178, and an OLED 180.

The gate of the switch TFT 170 and the gate of the switch TFT 172 areconnected to a select line Vsel. The first terminal of the switch TFT172 is connected to a data line Idata while the first terminal of theswitch TFT 170 is connected to the second terminal of the switch TFT 172which is connected to the gate of the reference TFT 174 and the gate ofthe dive TFT 176. The second terminal of the switch TFT 170 is connectedto the first terminal of the reference TFT 174. The capacitor 178 isconnected between the gate of the dive TFT 176 and ground. The firstterminal of the dive TFT 176 is connected to voltage supply Vdd. Thesecond terminal of the reference TFT 174 and the second terminal of thedrive TFT 176 are connected to the OLED 180.

When programming the pixel circuit 114B, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 172 and charges the capacitor 178. As thecapacitor voltage rises, the TFT 174 begins to turn on and Idata startsto flow through the TFTs 170 and 174 and OLED 180 to ground. Thecapacitor voltage stabilizes at the point when all of Idata flowsthrough the TFTs 170 and 174. The current flowing through the TFT 174 ismirrored in the drive TFT 176. In the pixel circuit 114B, by settingVsel to high and putting a voltage on Idata, the current flowing intothe Idata node can be measured. Alternately, by setting Vsel to high andputting a current on Idata, the voltage at the Idata node can bemeasured. As the TFTs degrade, the measured voltage (or current) willchange, allowing a measure of the degradation to be recorded. It isnoted that unlike the pixel circuit 114A of FIG. 5B, the current nowflows through the OLED 180. Therefore the measurement made at the Idatanode is now partially related to the OLED voltage, which will degradeover time. In the pixel circuit 114B, the analog voltage/current 112shown in FIG. 6B is connected to the Idata node. The measurement of thevoltage or current can occur anywhere along the connection between thedata driver IC 110 and the TFTs 116.

Referring to FIGS. 3B, 4B, and 6B, the pixel circuit 114 may allow thecurrent out of the TFTs 116 to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may allow somepart of the OLED efficiency to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may also allowa node to be charged, and the measurement may be the time it takes forthis node to discharge. The pixel circuit 114 may allow any parts of itto be electrically measured. Also, the discharge/charge level during agiven time can be used for aging detection.

Referring to FIG. 8B, an example of modules for the compensation schemeapplied to the system of FIG. 4B is described. The compensationfunctions module 130 of FIG. 8B includes an analog/digital (A/D)converter 140. The A/D converter 140 converts the measured TFTdegradation data 132 into digital measured TFT voltage/current 112 shownin FIG. 4B is connected to the Idata node. The measurement of thevoltage or current can occur anywhere along the connection between thedata driver IC 110 and the TFTs 116.

In FIG. 4B, the TFT-to-pixel circuit conversion algorithm is applied tothe measurement 132 from the TFTs 116. However, current/voltageinformation read from various places other than TFTs 116 may be usable.For example, the OLED voltage 122 may be included with the measured TFTdegradation data 132.

FIG. 6B illustrates a further example of the system 100 of FIG. 3B. Thesystem 100 of the FIG. 6B measured the OLED voltage 122. Thus, themeasured data 132 is related to the TFT 116 and OLED 120 degradation(“measured TFT and OLED voltage degradation data 132A” in FIG. 6B). Thecompensation functions module 130 of FIG. 6B implements the TFT-to-pixelcircuit conversion algorithm 134 on the signal related to both the TFTdegradation and OLED degradation. The TFT-to-pixel circuit conversionalgorithm module 134 or the combination of the TFT-to-pixel circuitconversion algorithm module 134 and the digital data processor 106estimates the degradation of the entire pixel circuit based on the TFTdegradation and the OLED degradation. The TFT degradation and OLEDdegradation may be measured separately and independently.

FIG. 7B illustrates an example of the pixel circuit 114 of FIG. 6B. Thepixel circuit 114B of FIG. 7B is a 4-T pixel circuit. The pixel circuit114B includes a switching circuit having TFTs 170 and 172, a referenceTFT 174, a drive TFT 176, a capacitor 178, and an OLED 180.

The gate of the switch TFT 170 and the gate of the switch TFT 172 areconnected to a select line Vsel. The first terminal of the switch TFT172 is connected to a data line Idata while the first terminal of theswitch TFT 170 is connected to the second terminal of the switch TFT172, which is connected to the gate of the reference TFT 174 and thegate of the drive TFT 176. The second terminal of the switch TFT 170 isconnected to the first terminal of the reference TFT 174. The capacitor178 is connected between the gate of the drive TFT 176 and ground. Thefirst terminal of the drive TFT 176 is connected to voltage supply Vdd.The second terminal of the reference TFT 174 and the second terminal ofthe drive TFT 176 are connected to the OLED 180.

When programming the pixel circuit 114B, Vsel is high and a voltage orcurrent is applied to the data line Idata. The data Idata initiallyflows through the TFT 172 and charges the capacitor 178. As thecapacitor voltage rises, the TFT 174 begins to turn on and Idata startsto flow through the TFTs 170 and 174 and OLED 180 to ground. Thecapacitor voltage stabilizes at the point when all of Idata flowsthrough the TFTs 152 and 154. The current flowing through the TFT 154 ismirrored in the drive TFT 156. In the pixel circuit 114A, by settingVsel to high and putting a voltage on Idata, the current flowing intothe Idata node can be measured. Alternately, by setting Vsel to high andputting a current on Idata, the voltage at the Idata node can bemeasured. As the TFTs degrade, the measured voltage (or current) willchange, allowing a measure of the degradation to be recorded. It isnoted that unlike the pixel circuit 114A of FIG. 5B, the current nowflows through the OLED 180. Therefore the measurement made at the Idatanode is now partially related to the OLED voltage, which will degradeover time. In the pixel circuit 114B, the analog voltage/current 112shown in FIG. 6B is connected to the Idata node. The measurement of thevoltage or current can occur anywhere along the connection between thedata driver IC 110 and the TFTs 116.

Referring to FIGS. 3B, 4B, and 6B, the pixel circuit 114 may allow thecurrent out of the TFTs 116 to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may allow somepart of the OLED efficiency to be measured, and to be used as themeasured TFT degradation data 132. The pixel circuit 114 may also allowa node to be charged, and the measurement may be the time it takes forthis node to discharge. The pixel circuit 114 may allow any parts of itto be electrically measured. Also, the discharge/charge level during agiven time can be used for aging detection.

Referring to FIG. 8B, an example of modules for the compensation schemeapplied to the system of FIG. 4B is described. The compensationfunctions module 130 of FIG. 8B includes an analog/digital (A/D)converter 140. The A/D converter 140 converts the measured TFTdegradation data 132 into digital measured TFT degradation data 132B.The digital measured TFT degradation data 132B is converted into thecalculated pixel circuit degradation data 136 at the TFT-to-pixelcircuit conversion algorithm module 134. The calculated pixel circuitdegradation data 136 is stored in a lookup table 142. Since measuringTFT degradation data from some pixel circuits may take a long time, thecalculated pixel circuit degradation data 136 is stored in the lookuptable 142 for use.

In FIG. 8B, the TFT-to-pixel circuit conversion algorithm 134 is adigital algorithm. The digital TFT-to-pixel circuit conversion algorithm134 may be implemented, for example, on a microprocessor, an FPGA, aDSP, or another device, but not limited to these examples. The lookuptable 142 may be implemented using memory, such as SRAM or DRAM. Thismemory may be in another device, such as a microprocessor or FPGA, ormay be an independent device.

The calculated pixel circuit degradation data 136 stored in the lookuptable 142 is always available for the digital data processor 106. Thus,the TFT degradation data 132 for each pixel does not have to be measuredevery time the digital data processor 106 needs to use the data. Thedegradation data 132 may be measured infrequently (for example, onceevery 20 hours, or less). Using a dynamic time allocation for thedegradation measurement is another case, more frequent extraction at thebeginning and less frequent extraction after the aging gets saturated.

The digital data processor 106 may include a compensation module 144 fortaking input luminance data for the pixel circuit 114 from the videosource 102, and modifying it based on degradation data for that pixelcircuit or other pixel circuit. In FIG. 8B, the module 144 modifiesluminance data using information from the lookup table 142.

It is noted that the configuration of FIG. 8B is applicable to thesystem of FIGS. 3B and 6B. It is noted that the lookup table 142 isprovided separately from the compensating functions module 130, however,it may be in the compensating functions module 130. It is noted that thelookup table 142 is provided separately from the digital data processor106, however, it may be in the digital data processor 106.

One example of the lookup table 142 and the module 144 of the digitaldata processor 106 is illustrated in FIG. 9B. Referring to FIG. 9B, theoutput of the TFT-to-pixel circuit conversion algorithm module 134 is aninteger value. This integer is stored in a lookup table 142A(corresponding to 142 of FIG. 8B). Its location in the lookup table 142Ais related to the pixel's location on the AMOLED display. Its value is anumber, and is added to the digital luminance data 104 to compensate forthe degradation.

For example, digital luminance data may be represented to use 8-bits(256 values) for the brightness of a pixel. A value of 246 may representmaximum luminance for the pixel. A value of 128 may representapproximately 50% luminance. The value in the lookup table 142A may bethe number that is added to the luminance data 104 to compensate for thedegradation. Therefore, the compensation module (144 of FIG. 7B) in thedigital data processor 106 may be implemented by a digital adder 144A.It is noted that digital luminance data may be represented by any numberof bits, depending on the driver IC used (for example, 6-bit, 8-bit,10-bit, 14-bit, etc.).

In FIGS. 3B, 4B, 6B, 8B, and 9B, the TFT-to-pixel circuit conversionalgorithm module 134 has the measured TFT degradation data 132 or 132Aas an input, and the calculated pixel circuit degradation data 136 as anoutput. However, there may be other inputs to the system to calculatecompensation data as well, as shown in FIG. 10B. FIG. 10B illustrates anexample of inputs to the TFT-to-pixel circuit conversion algorithmmodule 134. In FIG. 10B, the TFT-to-pixel circuit conversion algorithmmodule 134 processes the measured data (132 of FIGS. 3B, 4B, 8B, and 9B;132A of FIG. 5B; 132B of FIGS. 8B and 9B) based on additional inputs 190(e.g. temperature, other voltages, etc.), empirical constants 192, orcombinations thereof.

The additional inputs 190 may include measured parameters such as avoltage reading from current-programming pixels and a current readingfrom voltage-programming pixels. These pixels may be different from apixel circuit from which the measured signal is obtained. For example, ameasurement is taken from a “pixel under test” and is used incombination with another measurement from a “reference pixel.” Asdescribed below, in order to determine how to modify luminance data to apixel, data from other pixels in the display may be used. The additionalinputs 190 may include light measurements, such as measurement of anambient light in a room. A discrete device or some kind of teststructure around the periphery of the panel may be used to measure theambient light. The additional inputs may include humidity measurements,temperature readings, mechanical stress readings, other environmentalstress readings, and feedback from test structures on the panel

It may also include empirical parameters 192, such as the brightnessloss in the OLED due to decreasing efficiency (L), the shift in OLEDvoltage over time (Voled), dynamic effects of Vt shift, parametersrelated to TFT performance such as Vt, Vt, mobility ( ), inter-pixelnon-uniformity, DC bias voltages in the pixel circuit, changing gain ofcurrent-mirror based pixel circuits, short-term and long-term basedshifts in pixel circuit performance, pixel-circuit operating voltagevariation due to IR-drop and ground bounce.

Referring to FIGS. 8B and 9B, the TFT-to-pixel-circuit conversionalgorithm in the module 134 and the compensation algorithm 144 in thedigital data processor 106 work together to convert the measured TFTdegradation data 132 into a luminance correction factor. The luminancecorrection factor has information about how the luminance data for agiven pixel is to be modified, to compensate for the degradation in thepixel.

In FIG. 9B, the majority of this conversion is done by theTFT-to-pixel-circuit conversion algorithm module 134. It calculates theluminance correction values entirely, and the digital adder 144A in thedigital data processor 106 simply adds the luminance correction valuesto the digital luminance data 104. However, the system 100 may beimplemented such that the TFT-to-pixel circuit conversion algorithmmodule 134 calculates only the degradation values, and the digital dataprocessor 106 calculates the luminance correction factor from that data.The TFT-to-pixel circuit conversion algorithm 134 may employ fuzzylogic, neural networks, or other algorithm structures to convert thedegradation data into the luminance correction factor.

The value of the luminance correction factor may allow the visible lightto remain constant, regardless of the degradation in the pixel circuit.The value of the luminance correction factor may allow the luminance ofdegraded pixels not to be altered at all; instead, the luminance of thenon-degraded pixels to be decreased. In this case, the entire displaymay gradually lose luminance over time, however the uniformity may behigh.

The calculation of a luminance correction factor may be implemented inaccordance with a compensation of non-uniformity algorithm, such as aconstant brightness algorithm, a decreasing brightness algorithm, orcombinations thereof. The constant brightness algorithm and thedecreasing brightness algorithm may be implemented on the TFT-to-pixelcircuit conversion algorithm module (e.g. 134 of FIG. 3B) or the digitaldata processor (e.g. 106 of FIG. 3B). The constant brightness algorithmis provided for increasing brightness of degraded pixels so as to matchnondegraded pixels. The decreasing brightness algorithm is provided fordecreasing brightness of non-degraded pixels 244 so as to match degradedpixels. These algorithm may be implemented by the TFT-to-pixel circuitconversion algorithm module, the digital data processor (such as 144 ofFIG. 8B), or combinations thereof. It is noted that these algorithms areexamples only, and the compensation of non-uniformity algorithm is notlimited to these algorithms.

Referring to FIGS. 11A-1, 11B-1, 11C-1, 11D-1, and 11E-1, theexperimental results of the compensation of non-uniformity algorithmsare described in detail. Under the experiment, an AMOLED displayincludes a plurality of pixel circuits, and is driven by a system asshown in FIGS. 3B, 4B, 6B, 8B and 9B. It is noted that the circuitry todrive the AMOLED display is not shown in FIGS. 11A-1 through 11E-1.

FIG. 11A-1 schematically illustrates an AMOLED display 240 which startsoperating (operation period t=0 hour). The video source (102 of FIGS.3B, 4B, 7B, 8B and 9B) initially outputs maximum luminance data to eachpixel. No pixels are degraded since the display 240 is new. The resultis that all pixels output equal luminance and thus all pixels showuniform luminance.

Next, the video source outputs maximum luminance data to some pixels inthe middle of the display as shown in FIG. 11B-1. FIG. 11B-1schematically illustrates the AMOLED display 240 which has operated fora certain period where maximum luminance data is applied to pixels inthe middle of the display. The video source outputs maximum luminancedata to pixels 242, while it outputs minimum luminance data (e.g. zeroluminance data) to pixels 244 around the outside of the pixels 242. Itmaintains this for a long period of time, for example 1000 hours. Theresult is that the pixels 242 at maximum luminance will have degraded,and the pixels 244 at zero luminance will have no degradation.

At 1000 hours, the video source outputs maximum luminance data to allpixels. The results are different depending on the compensationalgorithm used, as shown in FIGS. 11C-1 through 11E-1.

FIG. 11C-1 schematically illustrates the AMOLED display 240 to which nocompensation algorithm is applied. As shown in FIG. 11C-1, if there wasno compensation algorithm, the degraded pixels 242 would have a lowerbrightness than the non-degraded pixels 244.

FIG. 11D-1 schematically illustrates the AMOLED display 240 to which theconstant brightness algorithm is applied. The constant brightnessalgorithm is implemented for increasing luminance data to degradedpixels, such that the luminance data of the degraded pixels 242 matchesthat of non-degraded pixels 244. For example, the increasing brightnessalgorithm provides increasing currents to the stressed pixels 242, andconstant current to the unstressed pixels 244. Both degraded andnon-degraded pixels have the same brightness. Thus, the display 240 isuniform. Differential aging is compensated, and brightness ismaintained, however more current is required. Since the current to somepixels is being increased, this will cause the display to consume morecurrent over time, and therefore more power over time because powerconsumption is related to the current consumption.

FIG. 11E-1 schematically illustrates the AMOLED display 240 to which thedecreasing brightness algorithm is applied. The decreasing brightnessalgorithm decreases luminance data to non-degraded pixels, such that theluminance data of the non-degraded pixels 244 match that of degradedpixels 242. For example, the decreasing brightness algorithm providesconstant OLED current to the stressed pixels 242, while decreasingcurrent to the unstressed pixels 244. Both degraded and non-degradedpixels have the same brightness. Thus, the display 240 is uniform.Differential aging is compensated, and it requires a lower Vsupply,however brightness decrease over time. Because this algorithm does notincrease the current to any of the pixels, it will not result inincreased power consumption.

Referring to FIG. 3B, components, such as the video source 102 and thedata driver IC 110, may use only 8-bits, or 256 discrete luminancevalues. Therefore if the video source 102 outputs maximum brightness (aluminance value of 255), there is no way to add any additionalluminance, since the pixel is already at the maximum brightnesssupported by the components in the system. Likewise, if the video source102 outputs minimum brightness (a luminance value of 0), there is no wayto subtract any luminance. The digital data processor 106 may implementa grayscale compression algorithm to reserve some grayscales. FIG. 12Billustrates an implementation of the digital data processor 106 whichincludes a grayscale compression algorithm module 250. The grayscalecompression algorithm 250 takes the video signal 104 represented by 256luminance values (251), and transforms it to use less luminance values(252). For example, instead of minimum brightness represented bygrayscale 0, minimum brightness may be represented by grayscale 50.Likewise, maximum brightness may be represented by grayscale 200. Inthis way, there are some grayscales reserved for future increase (254)and decrease (253). It is noted that the shift in grayscales does notreflect the actual expected shift in grayscales.

According to the embodiments of the present invention, the scheme ofestimating (predicting) the degradation of the entire pixel circuit andgenerating a luminance correction factor ensures uniformities in thedisplay. According to embodiments of the present invention, the aging ofsome components or entire circuit can be compensated, thereby ensuringuniformity of the display.

According to the embodiments of the present invention, the TFT-to-pixelcircuit conversion algorithm allows for improved display parameters, forexample, including constant brightness uniformity and color uniformityacross the panel over time. Since the TFT-to-pixel circuit conversionalgorithm takes in additional parameters, for example, temperature andambient light, any changes in the display due to these additionalparameters may be compensated for.

The TFT-to-Pixel circuit conversion algorithm module (134 of FIGS. 3B,4, 6, 8 and 9), the compensation module (144 of FIG. 8B, 144A of FIG.9B, the compensation of non-uniformity algorithm, the constantbrightness algorithm, the decreasing brightness algorithm and thegrayscale compression algorithm may be implemented by any hardware,software or a combination of hardware and software having the abovedescribed functions. The software code, instructions and/or statements,either in its entirety or a part thereof, may be stored in a computerreadable memory. Further, a computer data signal representing thesoftware code, instructions and/or statements, which may be embedded ina carrier wave may be transmitted via a communication network. Such acomputer readable memory and a computer data signal and/or its carrierare also within the scope of the present invention, as well as thehardware, software and the combination thereof.

Referring again to FIG. 3B, which illustrates the operation of the lightemitting display system 100 by applying a compensation algorithm todigital data 104. In particular, FIG. 3B illustrates the operation of apixel in an active matrix organic light emitting diode (AMOLED) display.The display system 100 includes an array of pixels. The video source 102includes luminance input data for the pixels. The luminance data is sentin the form of digital input data 104 to the digital data processor 106.The digital input data 104 can be eight-bit data represented as integervalues existing between 0 and 255, with greater integer valuescorresponding to higher luminance levels. The digital data processor 106can optionally manipulate the digital input data 104 by, for example,scaling the resolution of the video source 102 to a native screenresolution, adjusting the color balance, or applying a gamma correctionto the video source 102. The digital data processor 106 can also applydegradation corrections to the digital input data 104 based ondegradation data 136. Following the manipulations, the digital dataprocessor 106 sends the resulting digital data 108 to the data driverintegrated circuit (IC) 110. The data driver IC 110 converts the digitaldata 108 into the analog voltage or current output 112. The data driverIC 110 can be implemented, for example, as a module including a digitalto analog converter. The analog voltage or current 112 is provided tothe pixel circuit 114. The pixel circuit 114 can include an organiclight emitting diode (OLED) and thin film transistors (TFTs). One of theTFTs in the pixel circuit 114 can be a drive TFT that applies a drivecurrent to the OLED. The OLED emits visible light 126 responsive to thedrive current flowing to the OLED. The visible light 126 is emitted witha luminance related to the amount of current flowing to the OLED throughthe drive TFT.

In a configuration where the analog voltage or current 112 is aprogramming voltage, the drive TFT within the pixel circuit 114 cansupply the OLED according to the analog voltage or current 112 by, forexample, biasing the gate of the drive TFT with the programming voltage.The pixel circuit 114 can also operate where the analog voltage orcurrent 112 is a programming current applied to each pixel rather than aprogramming voltage. A display system 100 utilizing programming currentscan use current mirrors in each pixel circuit 114 to apply a drivecurrent to the OLED through the drive TFT according to the programmingcurrent applied to each pixel.

The luminance of the emitted visible light 126 is affected by aspectswithin the pixel circuit 114 including the gradual degradation ofhardware within the pixel circuit 114. The drive TFT has a thresholdvoltage, and the threshold voltage can change over time due to aging andstressing of the drive TFT. The luminance of the emitted visible light126 can be influenced by the threshold voltage of the drive TFT, thevoltage drop across the OLED, and the efficiency of the OLED. Theefficiency of the OLED is a ratio of the luminance of the emittedvisible light 126 to the drive current flowing through the OLED.Furthermore, the degradation can generally be non-uniform across thedisplay system 100 due to, for example, manufacturing tolerances of thedrive TFTs and OLEDs and differential aging of pixels in the displaysystem 100. Non-uniformities in the display 100 are generally referredto as display mura or defects. In a display 100 with an array of OLEDshaving uniform light emitting efficiency and threshold voltages drivenby TFTs having uniform gate threshold voltages, the luminance of thedisplay will be uniform when all the pixels in the display areprogrammed with the same analog voltage or current 112. However, as theOLEDs and TFTs in each pixel age and the degradation characteristicschange, the luminance of the display ceases to be uniform whenprogrammed the same.

The degradation can be compensated for by increasing the amount of drivecurrent sent through the OLED in the pixel circuit 114. According to animplementation of the present disclosure, compensation for thedegradation of the display 100 can be carried out by adjusting thedigital data 108 output from the digital data processor 106. The digitaldata processor 106 receives the degradation data 136 from thecompensation module 130. The compensation module 130 receivesdegradation data 132 based on measurements of parameters within thepixel circuit 114. Alternatively, the degradation data 132 sent to thecompensation module 130 can be based on estimates of expectedperformance of the hardware aspects within the pixel circuit 114. Thecompensation module 130 includes the module 134 for implementing thealgorithm 134, such as the TFT-to-pixel circuit conversion algorithm.The degradation data 132 can be electrical data that represents how mucha hardware aspect of the pixel circuit 114 has been degraded. Thedegradation data 132 measured or estimated from the pixel circuit 114can represent one or more characteristics of the pixel circuit 114.

In a configuration where the analog voltage or current 112 is aprogramming voltage, the programming voltage is generally determined bythe digital input data 104, which is converted to a voltage in the datadriver IC 110. The present disclosure provides a method of compensatingfor non-uniform characteristics in each pixel circuit 114 that affectthe luminance of the emitted visible light 126 from each pixel.Compensation is performed by adjusting the digital input data 104 in thedigital data processor 106 before the digital data 108 is passed to thedata driver IC 110.

FIG. 13B is a data flow chart showing the compression and compensationof luminosity input data 304 used to drive an AMOLED display. The dataflow chart shown in FIG. 13B includes a digital data processor block 306that can be considered an implementation of the digital data processor106 shown in FIG. 3B. Referring again to FIG. 13B, a video sourceprovides the luminosity input data 304. The input data 304 is a set ofeight-bit integer values. The input data 304 includes integer valuesthat exist between 0 and 255, with the values representing 256 possibleprogrammable luminosity values of the pixels in the AMOLED display. Forexample, 255 can correspond to a pixel programmed with maximumluminance, and 127 can correspond to a pixel programmed with roughlyhalf the maximum luminance. The input data 304 is similar to the digitalinput data 104 shown in FIG. 3B. Referring again to FIG. 13B, the inputdata 304 is sent to the digital data processor block 304. In the digitaldata processor block 304, the input data 304 is multiplied by four (310)in order to translate the eight-bit input data 304 to ten-bit resultingdata 312. Following the multiplication by four (310), the resulting data312 is a set of ten-bit integers existing between 0 and 1020.

By translating the eight-bit input data 304 to the ten-bit resultingdata 312, the resulting data 312 can be manipulated for compensation ofluminance degradation with finer steps than can be applied to theeight-bit input data 304. The ten-bit resulting data 312 can also bemore accurately translated to programming voltages according to a gammacorrection. The gamma correction is a non-linear, power law correctionas is appreciated in the art of display technology. Applying the gammacorrection to the input data can be advantageous, for example, toaccount for the logarithmic nature of the perception of luminosity inthe human eye. According to an aspect of the present disclosure,multiplying the input data 304 by four (310) translates the input data304 into a higher quantized domain. While the present disclosureincludes multiplying by four (310), in an implementation the input data304 can be multiplied by any number to translate the input data 310 intoa higher quantized domain. The translation can advantageously utilizemultiplication by a power of two, such as four, but the presentdisclosure is not so limited. Additionally, the present disclosure canbe implemented without translating the input data 304 to a higherquantized domain.

The resulting data 312 is multiplied by a compression factor, K (314).The compression factor, K, is a number with a value less than one.Multiplying the resulting data 312 by K (314) allows for scaling theten-bit resulting data 312 into compressed data 316. The compressed data316 is a set of ten-bit integers having values ranging from 0 to theproduct of K and 1020. Next, the compressed data 316 is compensated fordegradations in the display hardware (318). The compressed data 316 iscompensated by adding additional data increments to the integerscorresponding to the luminance of each pixel (318). The compensation fordegradation is performed according to degradation data 336 that is sentto the digital data processor block 306. The degradation data 336 isdigital data representing an amount of compensation to be applied to thecompressed data 316 within the digital data processor block 306according to degradations in the display hardware corresponding to eachpixel. Following the compensation for degradations (318), compensateddata 308 is output. The compensated data 208 is a set of ten-bit integervalues with possible values between 0 and 1023. The compensated data 308is similar in some respects to the digital data 108 output from thedigital data processor 106 in FIG. 3B. Referring again to FIG. 13B, thecompensated data 308 is supplied to a display driver, such as a displaydriver incorporating a digital to analog converter, to createprogramming voltages for pixels in the AMOLED display.

The degradations in the display hardware can be from mura defects(non-uniformities), from the OLED voltage drop, from the voltagethreshold of the drive TFT, and from changes in the OLED light emittingefficiency. The degradations in the display hardware each generallycorrespond to an additional increment of voltage that is applied to thepixel circuit in order to compensate for the degradations. For aparticular pixel, the increments of additional voltage necessary tocompensate for the hardware degradations can be referred to as:V_(mura), V_(Th), V_(OLED) and V_(efficiency). Each of the hardwaredegradations can be mapped to corresponding increments in data stepsaccording to a function of V_(mura), V_(Th), V_(OLED), V_(efficiency),D(V_(mura), V_(Th), V_(OLED), V_(efficiency)). For example, therelationship can be given by Expression 1: D(V_(mura), V_(Th), V_(OLED),V_(efficiency)) int[(2nBits−1) (V_(mura)+V_(Th)+V_(OLED)+V_(efficiency))V_(Max)], where nBits is the number of bits in the data set beingcompensated and V_(Max) is the maximum programming voltage. InExpression 1, int[ ] is a function that evaluates the contents of thebrackets and returns the nearest integer. The degradation data 336 sentto the digital data processor block 306 can be digital data createdaccording to the relationship for D(V_(mura), V_(Th), V_(OLED),V_(efficiency)) provided in Expression 1. In an implementation of thepresent disclosure, the degradation data 336 can be an array of digitaldata corresponding to an amount of compensation to be applied to thecompressed data of each pixel in an AMOLED display. The array of digitaldata is a set of offset increments that can be applied to the compresseddata by adding the offset increments to the compressed data of eachpixel or by subtracting the offset increments from the compressed dataof each pixel. The set of offset increments can generally be a set ofdigital data with entries corresponding to an amount of compensationneeded to be applied to each pixel in the AMOLED display. The amount ofcompensation can be the amount of increments in data steps needed tocompensate for a degradation according to Expression 1. In aconfiguration, locations in the array of the degradation data 336 cancorrespond to locations of pixels in the AMOLED display.

For example, Table 1 below provides a numerical example of thecompression of input data according to FIG. 13B. Table 1 providesexample values for a set of input data 304 following the multiplicationby four (310) and the multiplication by K (314). In the example providedin Table 1, K has a value of 0.75. In Table 1, the first column providesexample values of integer numbers in the set of input data 304. Thesecond column provides example values of integer numbers in the set ofresulting data 312 created by multiplying the corresponding input datavalues by four (310). The third column provides example values ofnumbers in the set of compressed data 316 created by multiplying thecorresponding values of the resulting data 312 by K, where K has anexample value of 0.75. The final column is the output voltagecorresponding to the example compressed data 316 shown in the thirdcolumn when no compensation is applied. The final column is created foran example display system having a maximum programming voltage of 18 V.In the numerical example illustrated in Table 1, the programming outputvoltage corresponding to the input data with the maximum input oftwo-hundred fifty-five is more than 4.5 V below the maximum voltage. The4.5 V can be considered the compensation budget of the display system,and can be referred to as the voltage headroom, Vheadroom. According toan aspect of the present disclosure, the 4.5 V is used to providecompensation for degradation of pixels in the AMOLED display.

TABLE 1 Numerical Example of Input Data Compression Output VoltageResulting Data Compressed Data (without degradation Input Data (×4)(×0.75) compensation) 255 1020 765 13.46 V 254 1016 762 13.40 V 253 1012759 13.35 V . . . . . . . . . . . . 2 8 6  0.10 V 1 4 3  0.05 V 0 0 0 0.00 V

According to an implementation of the present disclosure, the amount ofvoltage available for providing compensation degradation isV_(headroom). An amount of V_(headroom) can be advantageously reservedto compensate for a degradation of a pixel in an AMOLED display with themost severe luminance degradation. By reserving an amount ofV_(headroom) to compensate for the most severely degraded pixel, therelative luminosity of the display can be advantageously maintained. Therequired amount of V_(headroom) to compensate for the pixel in an AMOLEDdisplay with a maximum amount of degradation is given by Expression 2:V_(headroom)=max[V_(mura)+V_(Th)+V_(OLED)+V_(efficiency)]. In Expression2, V_(mura), V_(Th), V_(OLED), and V_(efficiency) can each be an arrayof values corresponding to the amount of additional voltage necessary tocompensate the pixels in the display, and the entries in the arrays ofvalues can correspond to individual pixels in the display. That is,V_(mura) can be an array of voltages required to compensate display muraor non-uniform defects; V_(Th) can be an array of voltage thresholds ofdrive TFTs of pixels in the display; V_(OLED) can be an array of OLEDvoltages of the pixels in the display; and V_(efficiency) can be anarray of voltages required to compensate for OLED efficiencydegradations of pixels in the display. In Expression 2, max[ ] is afunction evaluating an array of values in the brackets and returning themaximum value in the array.

As can be appreciated with reference to FIG. 13B and Table 1, the choiceof K affects the amount of V_(headroom) available to compensate fordegradations in the display. Choosing a lower value of K leads to agreater amount of V_(headroom). In a configuration of the presentdisclosure where the need for compensation increases over time due toaging of the display, the value of K can be advantageously decreasedover time according to the degradation of the display over time.Decreasing K enables uniformity compensation across the display suchthat pixels receiving the same digital input data actually emit lightwith the same luminance, but the uniformity compensation comes at thecost of overall luminance reduction for the entire display. FIGS. 14Bthrough 17B provide methods for selecting and adjusting K.

FIG. 14B is a flowchart illustrating a method for selecting thecompression factor according to display requirements and the design ofthe pixel circuit. In operation of the method illustrated by theflowchart in FIG. 14B, the display requirements and pixel circuit designof a display are analyzed to estimate maximum values of V_(mura),V_(Th), V_(OLED), and V_(efficiency) for the pixels in the display(405). The estimation (405) can be carried out based on, for example,empirical data from experimental results related to the aging ofdisplays incorporating pixel circuits similar to the pixel circuit inthe display 100. Alternatively, the estimation (405) can be carried outbased on numerical models or software-based simulation models ofanticipated performances of the pixel circuit in the display 100. Theestimation (405) can also account for an additional safety margin ofheadroom voltage to account for statistically predictable variationsamongst the pixel circuits in the display 100. Responsive to theestimation (405), the required voltage headroom is calculated (410). Therequired voltage headroom, V_(headroom), is calculated according toExpression 2. Once V_(headroom) is calculated, the compression factor,K, is calculated (415) according to Expression 3:K=1−V_(headroom)/V_(Max), where V_(Max) is a maximum programming voltagefor the display 100. The compression factor, K, is then set (420) foruse in the compression and compensation algorithm, such as thecompression algorithm illustrated in the data flow chart in FIG. 13B.

FIG. 15B is a flowchart illustrating a method for selecting thecompression factor according to a pre-determined headroom adjustmentprofile. A headroom adjustment profile is selected (505). The firstblock 505 in the flowchart in FIG. 15B graphically illustrates threepossible headroom adjustment profiles as profile 1, profile 2, andprofile 3. The profiles illustrated are graphs of K versus time. Thetime axis can be, for example, a number of hours of usage of the display100. In all three profiles K decreases over time. By decreasing K overtime, an additional amount of voltage (V_(headroom)) is available forcompensation. The example profiles in the first block 505 includeprofile 1, which maintains K at a constant level until a time thresholdis reached and K decreases linearly with usage time thereafter. Profile2 is a stair step profile, which maintains K at a constant level for atime, and then decreases K to a lower value, when it is maintained untilanother time, at which point it is decreased again. Profile 3 is alinear decrease profile, which provides for K to gradually decreaselinearly with usage time. The profile can be selected by a user profilesetting according to a user's preferences for the compensationtechniques employed over the life of the display. For example, a usermay want to maintain an overall maximum luminance for the display for aspecific amount of usage hours before dropping the luminance. Anotheruser may be fine with gradually dropping the luminance from thebeginning of the display's lifetime.

Once an headroom adjustment profile is selected (505), the display usagetime is monitored (510). At a given usage time, the value of thecompression factor, K, is determined according to the usage time andselected profile (515). The compression factor, K, is then set (520),and the display usage time continues to be monitored (510). After K isset (520), K can be used in the compression and compensation algorithm,such as the compression algorithm illustrated in the data flow chart inFIG. 13B. According to an aspect of the present disclosure, the methodof setting and adjusting K shown in FIG. 15B is a dynamic method ofsetting and adjusting K, because the value of K is updated over timeaccording to the usage time of the display 100.

FIG. 16B is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a threshold over a previous compensation. Measurements aretaken from aspects of the pixel circuits of the pixels in the display100 to measure V_(mura), V_(Th), V_(OLED), and V_(efficiency) (605) andcompute V_(headroom) according to Expression 2. The difference betweenthe value of V_(headroom) presently computed at time t2 is then comparedto the value of V_(headroom) computed at an earlier time t1 by computingthe difference (610). The difference is ΔV_(headroom), and is calculatedaccording to Expression 5:ΔV_(headroom)−(V_(headroom))_(t2)−(V_(headroom))_(t1). In Expression 5,t1 is the last time used to adjust the compensation factor, K, and t2 isa present time. The subscripts in the right hand side of Expression 5indicate a time of evaluation of the quantity in parentheses.

The calculated value of ΔV_(headroom) is then compared to a compensationthreshold, V_(thresh) (615). If ΔV_(headroom) exceeds V_(thresh), K ismodified (620). If ΔV_(headroom) is less than or equal to V_(thresh), Kis not modified. The value of K can be modified according to Expression6: K_(new)=K_(old)/A−B, where K_(new) is the new value of K, K_(old) isthe old value of K, and A and B are values set for applications anddifferent technologies. For example, A and B can be set based onempirical results from experiments examining the characteristicdegradation due to aging of pixel circuits similar to those used in thedisplay 100 to drive OLEDs in each pixel. Similar measurements or userinputs can be used to set V_(thresh) as well. The compression factor, K,is then set (625) for use in the compression and compensation algorithm,such as the compression algorithm illustrated in the data flow chart inFIG. 13B. Degradation measurements continue to be measured (605),ΔV_(headroom) continues to be calculated (610), and K is updatedaccording to Expression 6 whenever ΔV_(headroom) exceeds V_(thresh)(620). According to an aspect of the present disclosure, the method ofadjusting K shown in FIG. 16B is a dynamic method of adjusting K,because the value of K is updated over time according to degradationmeasurements gathered from the pixel circuits within the display 100.

Alternatively, the compression factor can be modified (620) according toExpression 3 based on the measured V_(headroom). According to an aspectof the method provided in the flowchart shown in FIG. 16B, the value ofK is maintained until a threshold event occurs (615), when K is modified(620). Implementing the method provided in FIG. 16B for adjusting thecompression factor, K, can result in K being decreased over timeaccording to a stair step profile.

FIG. 17B is a flowchart illustrating a method for selecting thecompression factor according to dynamic measurements of degradation dataexceeding a previously measured maximum. Measurements are taken fromaspects of the pixel circuits of the pixels in the display 100 tomeasure V_(mura), V_(Th), V_(OLED), and V_(efficiency) (605). Themeasurements of V_(mura), V_(Th), V_(OLED), and V_(efficiency) arereferred to as degradation measurements. The maximum values of thedegradation measurements are selected (710). The maximum values of thedegradation can be selected according to Expression 2. The combinationof measuring the degradation measurements (605) and selecting themaximum values (710) provides for ascertaining the maximum compensationapplied to pixels within the display. The maximum values are compared topreviously measured maximum values of previously measured degradationmeasurements (715). If the presently measured maximum values exceed thepreviously measured maximum values, V_(headroom) is calculated accordingto Expression 2 (410) based on the present degradation measurements.Next, the compression factor, K, is determined according to Expression 3(720). The compression factor is set (725) and the maximum values areupdated for comparison with new maximum values (715). The compressionfactor is set (725) for use in the compression and compensationalgorithm, such as the compression algorithm illustrated in the dataflow chart in FIG. 13B. Similar to the method provided in FIG. 16B, themethod shown illustrated by the flowchart in FIG. 17B is a dynamicmethod of adjusting K based on degradation measurements continuallygathered from the pixel circuits within the display 100.

The present disclosure can be implemented by combining the abovedisclosed methods for setting and adjusting the compression factor, K,in order to create an adequate amount of voltage headroom that allowsfor compensation to be applied to the digital data before it is passedto the data driver IC. For example, a method of setting and adjusting Kaccording to FIG. 16B or FIG. 17B can also incorporate a user selectedprofile as in FIG. 15B.

In an implementation of the present disclosure, the methods of selectingand adjusting the compression factor, K, provided in FIGS. 14B through17B can be used in conjunction with the digital data manipulationsillustrated in FIG. 13B to operate a display while maintaining theuniform luminosity of the display. In a configuration, the abovedescribed methods allow for maintaining the relative luminosity of adisplay by compensating for degradations to pixels within the display.In a configuration, the above described methods allow for maintainingthe luminosity of a pixel in a display array for a given digital inputby compensating for degradations within the pixel's pixel circuit.

FIG. 18B is a flow chart illustrating a method of periodically adjustingthe peak luminance for compensation. The initial peak luminance set bythe display at step 801 is adjusted based on compensation levels at step802. After calculating the compensated value for each pixel to providethe peak brightness at step 803, the number of pixels whose values arelarger than a threshold voltage is calculated at step 804. If thisnumber is larger a threshold number (threshold_error), the peakluminance (brightness) is reduced at step 805 until the number is lessthan threshold_error.

1. Initial brightness can be set by applications or an algorithm thatcontrols the power, temperature, or any other display factors.

2. The pixel values can be the data passed to the display driver, thepixel luminance or the pixel currents. One can calculate more than onepixel value to compare with more than one threshold value.

3. The threshold values can be set based on different conditions such asthe maximum compensated headroom available and aging accelerationfactors. For example, as the current of the pixel is increased tocompensate for the OLED aging, the OLED aging accelerates. Therefore,one can set a threshold value to limit the aging acceleration. Thethreshold values can be more than one and can be different for eachsub-pixel.

4. The threshold_error can be set as the maximum tolerable number ofpixels having the wrong compensation level. There can be differentthreshold_error values for different threshold (pixel) values.

5. In the case of multiple threshold values, there can be a prioritylist in which the conditions of the values with higher priority need tobe fixed first.

6. The compensation factors can include uniformity compensation, agingcompensation, temperature compensation, and other adjustments related todisplay performance.

7. The adjustment can be made periodically, at an event (e.g., power on,power off, readjusting the compensation factors, etc.) or at user(application) request.

FIG. 19B is a flow chart illustrating a method of periodically adjustingthe operating conditions for compensation. The initial operatingconditions (e.g., voltages, currents, gray levels, etc.) are set at step901, and the compensation factors for the pixels are calculated at step902. After calculating the pixel values for compensated peak brightnessat step 903, the number of pixels whose values are larger than athreshold value is calculated at step 904. If this number is larger thana threshold number (threshold_error), the operating conditions areadjusted at step 905 so that the number of pixels with values largerthan the threshold is less than threshold_error. Then at step 906 thethreshold values are re-adjusted based on the new voltage levels.

1. Initial operating conditions can be set by applications or analgorithm that controls the power, temperature, or any other displayfactors.

2. Pixel values can be the data passed to the display driver, the pixelluminance or the pixel currents. One can calculate more than one pixelvalue to compare with more than one threshold value.

3. The threshold values can be set based on different conditions such asthe maximum compensated headroom available.

4. The threshold_error can be set as the maximum tolerable pixels withwrong compensation levels. There can be different threshold_errors fordifferent threshold (pixel) values.

5. The compensation factors can include uniformity compensation, agingcompensation, temperature compensation, and other adjustments related todisplay performance.

6. In case of multiple threshold values, there can be a priority list inwhich the conditions of the values with higher priority need to be fixedfirst.

7. The adjustment can be made periodically, at an event (e.g., power on,power off, readjusting the compensation factors, etc.) or at user(application) request.

A combination of luminance adjustment and display operating conditions,i.e., a hybrid adjustment, may be used to meet the threshold_errorvalues.

1. In one case, different threshold values are allocated to differentparameters (e.g., some are allocated to the luminance adjustment andsome to the display operation conditions). For example, the agingacceleration factor threshold value can be allocated to the luminanceadjustment, and the uniformity value can be allocated to the displayoperation condition algorithm. Also, some threshold values can havepriority over others so that the higher priority values are fixed first.

2. In another case, there can be a percentage correction for eachparameter. For example, the maximum change in the luminance (or the rateof luminance reduction) can be limited. In this case, if there are somethreshold_errors left after adjusting the luminance according theallowable rate, they are fixed by the operation condition adjustment.

3. In another case, one can use a mixture of the two aforementionedcases (some threshold values are controlled by specific parameters(e.g., aging acceleration is controlled by a luminance adjustmentalgorithm), and some threshold values are allocated to both algorithms.

The present disclosure describes maintaining uniform luminosity of anAMOLED display, but the techniques presented are not so limited. Thedisclosure is applicable to a range of systems incorporating arrays ofdevices having a characteristic stimulated responsive to a data input,and where the characteristic is sought to be maintained uniformly. Forexample, the present disclosure applies to sensor arrays, memory cells,and solid state light emitting diode displays. The present disclosureprovides for modifying the data input that stimulates the characteristicof interest in order to maintain uniformity. While the presentdisclosure for compressing and compensating digital luminosity data tomaintain a luminosity of an AMOLED display is described as utilizingTFTs and OLEDs, the present disclosure applies to a similar apparatushaving a display including an array of light emitting devices.

The present invention has been described with regard to one or moreembodiments. However, it will be apparent to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A method of operating a display having aplurality of pixels to compensate for a degradation of the plurality ofpixels, wherein the plurality of pixels have driving circuits fordriving currents through light emitting devices based on inputs, themethod comprising: receiving a video signal having a set of grayscalevalues for the plurality of pixels; dynamically adjusting a grayscalecompression factor according to at least one input received after usageof the display, the grayscale compression factor greater than zero andless than one; compressing the video signal using the grayscalecompression factor to produce compressed grayscale values so as to usefewer luminance values than in the original video signal and reserving aremaining amount of grayscale values for future adjustments associatedwith aging; compensating said compressed grayscale values fordegradation of said display, using at least a portion of said reservedgrayscale values; and outputting the compensated compressed grayscalevalues to a display driver for programming the driving circuits of theplurality of pixels.
 2. The method of claim 1, wherein the set ofgrayscale values is eight-bit data, and wherein the compressing iscarried out according to a grayscale compression algorithm thattranslates the set of compressed grayscale values having a range of 200values.
 3. The method of claim 1, wherein the remaining amount ofgrayscale values are reserved at a high end of the available range toprovide adjustments that increase the luminosity of adjusted pixels. 4.The method of claim 1, wherein the remaining amount of grayscale valuesare reserved at a low end of the available range to provide adjustmentsthat decrease the luminosity of adjusted pixels.
 5. The method of claim1, further comprising: adjusting a value in the set of grayscale valuescorresponding to at least one of the plurality of pixels according to adecreasing brightness algorithm.
 6. The method of claim 1, furthercomprising: adjusting a value in the set of grayscale valuescorresponding to at least one of the plurality of pixels according to aconstant brightness algorithm.
 7. The method of claim 1, wherein saidcompensating said compressed grayscale values for degradation of saiddisplay includes compensating at least one of said compressed grayscalevalues for degradation of at least one of the plurality of pixels of thedisplay, each one of the at least one of said compressed grayscalevalues compensated for degradation of an associated respective one ofthe at least one of the plurality of pixels of the display.